I can't believe I read through this whole thread. Two years in the making. Very interesting opinions though! I got a kick out of how about halfway through everyone seemed to take a liking to GM's Syncromesh at balkamp's suggestion. Good deal.
I'm at 12,500 right now, but I think I'll just check the fluid at 15,000 and stick with the stock fluid until about 30,000 unless the stock stuff looks nasty.
So what's the recommendation for the rear diff? Will any synthetic 75W-90 of choice do the job?