have a 03 blazer 2wd clunks in the rear end only when you put it in reverse and only does it sometimes?

 

Put the parking brake on and the tranny in neutral (engine off of course)...
Crawl under there and grab ahold of the drive shaft. Rotate it back and forth as well as up / down and sideways. Check the U-joints and check for play in the rear-end pinion.

 

Probably a U-joint
Tranny may be on its way out.
Rear end take the cover off and inspect gears look at them closely.
Blazers are hard on spider gears.
I had mine blow the pinion gear it shatter in pieces, my spider still looked good imagine that.

 

You didn't mention how many miles are on it, but I would check the U-joints first.