Rear end clunking

Went over rail road crossing at low speed an a clunking sound started coming from rear of truck. I pulled over an saw nothing was hanging down. Started to pull off an clunk clunk clunk. Sound is more prevalent in reverse. Got towed to a buddy's and jacked it up grabbed driveshaft an twisted. It had lots movement so I replaced u-joint at tail end of shaft that goes to rear end. Pulled out drive an was not much different. The spline that goes in trans was dry and I did not put anything on it. Any suggestions are appreciated. My truck is a z71.