IMO, a lot depends on how much you use the hydraulic system and the climate. Remember that the actual viscosity of a 90 grade, measured in centistokes, is only a few steps higher than the specified oil.If you look at the main BITOG website home page, you'll see a viscosity grade chart that shows the actual viscosity of the different engine and gear oil grades and see the similarities. A 40 grade engine oil is the rough viscosity equivalent of a 90 grade gear oil.
Anyway, I know a number of local farmers with tractors like ours (I have an '89 2810 Ford as my utility tractor, which is an evolution of yours) that use a GL5 80W90 grade in the final drive. The 3-point can be a little sluggish when cold (and the hydraulic pump likely bypasses on a really cold day until it warms up a bit). If you don't use it much in cold weather, it may not be an issue. If you have a power-beyond setup, with hydraulic remotes, the heavier oil may be problematic for hydraulic equipment used with it, but not unworkable in most situations, especially on warm days.
I believe the 2000 has a separate final drive housing that is used as the hydraulic reservoir, and the trans and power steering (if equipped) are each separate. The "134" is perfect for the final drive and hydraulics. I was fairly unhappy with it in mine in the trans. I put 75W90 synthetic in the trans and it radically improved the shifting (bear in mind I did it to use up some aging stock more than anything. I'm too much of a tightwad to do that on purpose( : < ). I don't currently have 134 in my power steering, but a generic fluid of the same viscosity and general characteristics.
In my long winded way, my bottom line recommendation is that if you have time and miles to get out of the oil that's in there now, I wouldn't get too excited about dumping it by tomorrow since it's obviously worked OK for all these years. Lots of people do it. When it's time to change, however, go with the 134 in the final drive and power steering. If the trans is separate, that too.