My 05 LLY 4x4 with 130k has a lot of slack somewhere in the driveline. I thought it might be the splines needing greasing but I did that and it did nothing. If your off the gas then press it there is about a half second pause then it catches and makes a clunk. You can look under the truck and go from drive to reverese and it makes a loud clunk, but I can't tell if it is in the rearend or t case.
Anyway ive never driven something with this bad of slack in the driveline and its really annoying.