my friend has an 86 IROC-z with a 305 tpi automatic. he just bought it yesterday and while we were driving it he attemtped to go from a dead stop and it hooked up pretty good, but then clunked noticeably loud. Do you think that it is the motor mounts? or would it possibly have something to do with the rear end?

I'd guess tranny mount.

is this easy to replace? expensive/non-expensive?

$25 and under an hour of labor. Tranny mount is my guess as well.