Any suggestions on how to diagnose drivetrain noise?  I checked the U-joints
and they are not obviously worn or loose.  Can stiff U-joints cause noise?
I'm getting a loud whine only when accelerating--it goes away if I take my
foot off the gas.  This is a 1978 F350 Crew Cab with an 8' bed and about
95000 miles on it.  It sounds like the noise is coming from under the cab as
opposed to back by the rear axle.  It has a two-piece axle with a carrier
bearing in the middle.  I jacked up the rear, put it in N and wiggled and
turned stuff and didn't find anything obvious.  It's an automatic 3-speed
transmission (C6, I believe?).

I can't afford to spend any money on unneeded parts right now so any help
would be much appreciated.  If it is a differential bearing could the noise
still sound like it's coming from the middle of the truck?  If I remove the
driveshaft should I expect to hear anything indicating a bad differential
bearing by turning the wheels or whatever?