You only need friction modifier if your truck has a limited slip (LS) and but if it does, the synthetic oil may have the required friction characteristic with no additive needed. Also, some oils are sold with the right amount of FM already added (and they say on the jug).
Overall, you want to avoid adding it if you can, as the FMs tend to reduce the oxidative resistance of the oil. If your truck has a LS, put in the syn oil without a modifier, drive a few Figure-8s in a parking lot (which works the oil up into the clutches) and drive for a few days to see. If no chatter, great... go on about your biz. If chatter, add FM an ounce at a time, doing the parking lot thing between until the chatter stops or you reach the maximum amount listed for that axle (3 oz according to my book).
If it does have a LS, you will find a tag on the diff that has a number that indicates the axle ratio and the presence of a limited slip. For you F-350, the axle ratio is likely 3.73 or 4.10:1, so the tag might read "3L73" or "3.73L" if it has a limited slip with a 3.73 ratio and without the "L" whatever ratio without a limited slip. Your door sticker will also have the axle code under "Axle" or "Ax". Rather than list al the codes, if you want to know, go back and look and put the code back here and I'll look it up next time I'm here.
