I have a 99 4wd blazer and when I go to go forward or reverse I'm hearing a clunk sound from the rear of blazer. I'm also hearing it as I slwo down. when I get around 25/30 mph and I think it's when the tranny shifts down I can hear the clunk noise from the rear. I had my wife stand by the blazer and I would go forward and reverse and she could hear the clunk noise coming from the rear of the blazer. If I opened the door and leaned out I could also tell it was coming from the rear. As soon as I put it into gear either forward or reverse I get the loud clunk noise and when i'm slowing down around 25 mph I get the clunk sound. today I also believe I heard the clunk sound when I was starting to drive off and when the tranny shifted around 25mph I heard the clunk noise from the rear. It only makes the clunk noise one time and I don't hear it again until I slow down around 25 mph and when putting it into gear. As soon as I put it into gear I hear the clunk noise from the rear.

 

Sounds like its time for new u joints.