I have a 91 gmc sierra and the other night a clunking sound from somewhere in the rear is making a random clunking sound and it hard to find out where it's coming from. It only does it when I drive. I don't know if it's tranny or axle

 

You might try jacking up the rear end and running it to see if it makes the noise.

 

I jacked it up but it only made one clunk, and it was so fast I couldn't tell where it came from