Here's my advice, and the reasoning behind it ...
Front diff:
First you need to determine if the front diff has a vent cap, and if it does, which color the vent cap is (black or off-white). It should be on the driver side of the diff, above the drain plug. I say this because there was a fluid compatibility issue way back when your vehicle was made. However, this may not apply to your specific vehicle. You will have to determine if your vehicle would be effected. Here is a copy of the GM statement:
Document ID# 1548733
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Subject: Info - Use of Synthetic Front Axle Lubricant For 4WD Vehicles Sold in Cold Weather Climates #03-04-19-001B - (09/03/2004)
Models: 2002-2004 Chevrolet Avalanche 2500, Silverado 2500/3500, Suburban 2500
2002-2003 Chevrolet Silverado 1500HD
2002-2004 GMC Sierra 2500/3500, Yukon XL 2500
2002-2003 GMC Sierra 1500HD
with Four-Wheel Drive and 9.25" Front Axle Assemblies and Built Prior to April 1, 2004
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This bulletin is being revised to update vehicle applications, part numbers and fluid fill information. Please discard Corporate Bulletin Number 03-04-19-001A (Section 04 -- Driveline/Axle).
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Important: This bulletin is intended for vehicles sold in very cold weather climates, especially those in Alaska and certain regions of Canada.
Important: This bulletin DOES NOT apply to vehicles built after April 1, 2004. Vehicles referenced above that were built after April 1, 2004 were manufactured with front axles containing synthetic axle lubricant and do not need to have the front axle fluid inspected or changed.
Front axles in the above-listed vehicles built before April 1, 2004 are manufactured with mineral-oil base axle fluid. Because of the 9.25" front axle assembly's internal design characteristics, there may be insufficient lubrication flow at very low ambient temperatures (-24°C (-12°F) or below). This may contribute to premature pinion bearing wear if a vehicle is driven extensively in very low ambient temperatures.
Important: Please be certain that PDI personnel know to perform the drain/refill procedure all year round (not just in the cold weather months).
New vehicles that are expected to operate extensively in 4WD in temperatures below -24°C (-12°F) should have their front axle fluid drained and refilled with synthetic axle lubricant during PDI or before delivery to the first retail customer.
Follow the Front Drive Axle Lubricant Replacement procedure found in the Service Manual (SI Document ID #673354). It is not necessary to flush the front axle assembly. Use Synthetic Axle Lubricant, P/N 12378261 or P/N 89021677 (Canadian P/N 10953455 or P/N 89021678 ). Each of these part numbers is the same axle lubricant, with differences in packaging only. Fill to a level between 6 and 9 mm (0.25 in to 0.375 in) below the bottom of the fill plug.
For vehicles that have the axle lubricant changed, the front axle vent hose connector assembly will need to be inspected.
Important: Front axle assemblies with a vent hose connector assembly that is white in color do not need to be changed.
If the vent hose connector is black in color, it will need to be changed so that the O-ring seal is compatible with the synthetic lubricant. Install vent connector, P/N 12479390 . Follow the Front Drive Axle Vent Hose Connector Replacement procedure in the Service Manual (SI Document ID #843184).
Parts Information
Part Number Description Qty
12378261 or 89021677
(Canadian P/N 10953455 or 89021678 ) Oil, Synthetic Gear 2
12479390 Connector, Axle Vent 1
Parts are currently available from GMSPO.
Warranty Information
For vehicles repaired under warranty, use:
Labor Operation Description Labor Time
*F0011 Fluid, Axle and Differential (Front) - Drain and Fill 0.5 hr
Add To Replace Front Axle Vent Hose Connector 0.1 hr
*This is a unique labor operation number for use only with this bulletin. This number will not be published in the Labor Time Guide.
Document ID# 1548733
Looking that document over, you can see that there was a seal (0-ring) compatibility issue with the GM synthetic lube additives. Essentially, they used the vent cap color as an external visual clue to help ID what seal material was used in the diff. So, if you have a black vent cap, and intent to use the GM syntehtic fluid, that would be a bad idea! However, I have no ability to tell you if OTHER brands of syn fluids would have that same issue; at this point, you'd be own your own. But note that this tech bullentin is for the use of syn fluids in UBER-COLD regions, and if you don't live in that type area, then you don't need a syn to begin with. Therefore, any decent dino fluid would work well. I would recommend either Valvoline High Performance or Mobil Mobilube, either of them in 80w-90. That is because they both pass the very strict performance testing in SAE J2360 of GL-5 gear oils, which is the cilvilian equivilant of the MIL spec fluids. They are going to be able to protect every bit as well as a syn in nearly any application expect stupid-cold areas. If you don't have caribu running outside your windown, this probably will not be a concern for you. I generally would agree that a syn would be OK, but GM specifically has a note warning against the use of their syn which certain seals, so be forewarned. If you have a white cap, then any syn would be OK. If you don't have a white cap, I'd stick with a dino fluid, as mentioned.
Rear diff:
GM's fluid spec (9986115) is merely a designation of a syn 75w-90 with LS additive, and nothing more. There is a quote from a guy named Michael Torrenson (or something similar to that) who worked for Eaton, where he indicates that ANY GL-5 fluid will be OK for a GM G-80 diff, and the only concern is to make sure you use one with a LS additive, because the G-80 "locker" actually has a clutch engagement system. Just about any decent GL-5 lube will work here, but syn's are going to give you a longer OCI, so they might be worth the investment. Most of the syns, and some of the dinos, will have LS additive already included. Of note, the 80w-90 Valvoline does I know for sure, because I just put some in my Ford diff this weekend. If you live in super-hot areas, AND you town for the MAJORITY of the operational time, then I'd "upgrade" to a 85w-140. But ONLY if you meet BOTH of those criteria. If you don't, then use a 75w-90 syn or 80w-90 dino that meets SAE J2360. Adjust your OCI to your fluid type. If you have a G-80 locking diff (and most did) then you must use a LS compliant fluid. You will need to either buy a fluid with LS additive included, or put it in separately. Regardless of what others state, the G-80 does have clutches. They often mistakenly think that the "locker" is a mechanical type, but it is actually a pawl-operated clutch system. It has clutches. You need LS additive if you have a G-80.
Transfer case:
First, determine if you have an "Autotrak" transfer case; this is the one that automatically engages active traction control via locking and unlocking of the case. It is NOT the same thing as the simpler electronically shifted unit where the drive selects case mode. To be specific, they are both shifted electronically, but the difference is who decides when 4x4 is used; the driver or the computer. If you have an "Autotrak" x-fer case, you need to make sure to use an Autotrak compliant fluid. It's the blue colored GM fluid, and I'm not sure there are aftermarket options here. If you do not have an "Autotrak" t-case, then you can use any decent Dex/Merc ATF. Again, the use of syns here is going to get you a longer lifecycle on the drain interval. I would recommend a syn ATF such as Mobil 1 ATF, Amsoil ATF or ATD, etc and not have to change it for a LONG time. These ONLY apply if the case is NOT an Autotrack unit.
Hope that helps.