Chevy & GMC Vans

i've noticed that only when i'm backing up and cutting my wheels to the right, i hear a strange sorta clunking/wierd noise. i can't really describe it. it only seems to happen when i'm backing up. is there any tell tale sign of a bad cv joint? is it cheaper/easier to buy a new/used axle? it's pretty much the only thing i can think of that could be causing this noise. i need some advice.

thanks..

I know on FWD cars when turning and accelerating, if you get a clicking up front, the CV shaft is shot. I replaced both on the SPB after doing the lift because I though they might be the source of my vibrations. The rebuilt ones ended with the same result. I decided to swap them out because there was some play in them and I thought they were bad. They were not.