I once made this silly mistake after rebuilding a rear end: I forgot to add lubricant! That was a LOOOOOOOOOOOOOG time ago, however. Not to be insulting, but did YOU remember?
Eliminate the easy things first. And what grade did you use? 600w or a modern multi-viscosity?
After a drive, feel the pumpkin center. If too hot to keep your hand on the metal, the pre-load is too tight or there's not enough clearance between the pinion and ring gear teeth. You may notice the paint peeling off the center of the rear end because of the extra heat generated by these problems.
Marshall