Just finished installing the engine and trans in my '84 SD. It's good to have the oil leaks fixed but now I have this odd problem. The car shifts great but when put in N from R or P from any other gear I get a terrible shutter/clunk. Wasn't there before I removed the engine and trans. I am trying to post a video to show what is happening but am having trouble posting it.

Looking at the drive shaft while the car is running and someone moves it through the gear selections you can see the shaft "rattle" pretty well. New flex disc at the trans end. The one on the chunk end is fairly new too. The drive line seems tight. Trans shifts great as I stated earlier. I dunno. Gonna have to study on this one.