Maybe the t-case?

[JollyGreen]

Mine's doing the same thing. Apparently this is a problem with these trucks after a while. It only does it taking off in D as well. There is a nickel plated slip yoke you can buy, but you have to get new driveshaft as well. Here's a good fix. This is what I'm going to do when my truck is down getting the tranny fixed this spring.

http://www.fullsizechevy.com/forum/...ce/443340-solution-chevy-slip-yoke-clunk.html

 

[DGA1]

Thanks. This helps quite a bit and I'll plan on doing it next time I remove the driveshaft.