I broke a u joint and while underneath it I noticed my u bolts were broke so I replaced them as well. After putting it all back together I took it on a short test drive in the field. As I was coming up to the parking spot I heard a clunk so I stopped now rear tires are locked up and will not move. I have checked the rear brakes are not hung up so what am I missing.

 

Anything specific will depend on what type differential you have, but...
there's no doubt that damage done was not limited to your U-Joint.

Gonna have to open it up, and see what's hung up.

 

when you removed your driveshaft, did you make sure to re-grease up the end that goes into the transmission?
If it's not that end, it would be something with your differential.
When I did my Ujoints I changed my rear end fluid. I know it sucks, but open up the rear end of the differential and see what's going on in there.