For a while now my 91 RS has had a rattling problem in either the drive shaft or transmission. The trasmission was just rebuilt and the rear u-joint was replaced. Every time I am in gear and at a stop the drive shaft rattles really bad. Its not constant though, it will rattle for a second or so, then stop, then do it again until I start moving. It only does it in gear, it is fine in park and neutral. Any suggestions??

It maybe your converter bracket. it is attached to the tail shaft of your trans. very common problem.

Is this something that can be easily tightened or fixed?? Can I do it or do I need to take it back to the shop that rebuilt the tranny??

you can fix it. it is just ametal braket at the end of the trans make sure it is tight at the trans and where it ataches to your converter