1990 automatic with about 285k miles.
Transmission was rebuilt a few years ago. It shifts UP very smooth, even if I stomp on the pedal. But when I let off the pedal quickly and it kicks down into first gear, I get a pretty loud clunk. I get the same clunk whenever I shift into reverse.
What should I be looking for? Im thinking rear end, not a tranny issue?
thanks in advance

 

U joints?

 

At 285K the differential may need attention. The open diffs wear out the cross shafts and the limited slips wear out the clutches in both cases creating a lot of slop. I think there's some info in the sticky on this.

 

I concur. Park the truck on level ground so the weight of the truck doesn't roll against the Park pins in the tranny. Go underneath and rock the drive shaft. When I do this, I have about 1/10th of a revolution in the rear end! My Powr-Lok needs rebuild bad.

 

We had a half-ton '82 that had a bad clunk. When you parked it on a hill it would roll about 6 inches before it caught. The spider gear shaft slid out one day during a u-joint replacement and it would only move a couple feet at that point.

 

Well I climbed under the truck and it was pretty obvious what the problem was. I grabbed onto the driveshaft, gave it a twist, and it was flopping around bad. One of the u joint bearings was totally wasted.
Removed the drive shaft, replaced both u-joints, and no more clunk. thanks for the replys.