Back up from the dead, nearly 5 years ago - did you ever solve this as I have a very similar problem. In short, under power,(heavy power), big clunking/something clearly not right. The 4wd selector motor was new from last year and I can switch into 4wd low, so it is moving fine. I have replaced the front diff solenoid. Drained and replaced front diff and transfer case fluids. Seems like the clutches that let you engage 4wd hi at moderate speeds are slipping? Is that possible? I'll post a separate thread, but wanted to see if you figured it out. I have an 03 2500HD with the duramax. 4wd, lowered a little, so no lift.
'03 GMC 2500HD 4wd w/ Swapped LLY.