1978 Chevy 4x4 half ton clunkin noise

I've got a friend with a 78 1/2 4x4 chevy. If you are hauling or giving the truck a little gas you can here and feel a metal to metal thump. He drove and i stood outside the truck trying to see what was moving but nothing was thumping around even though we could here it. Any suggestions? i hope its not the front axle. If so bahumbug i guess. Needs fixed to haul racecar around.  

Check pinion bearings and u-joints. Thats the usually culprit on those year models  

+1 it is most likely in the rear end.  
Professional hi-performance engine builder.
Horsepower sells Engines and torque wins races.

U joints are fine. Thats the first thing i checked. Not sure about the pinion bearings yet though.