This is my situation. The point where the driveshaft goes into the transmission is slightly loose. You can wiggle it a little with your hand. When I hit about 55-60mph it vibrates some. What is the cause of this and does it take a lot to fix cost wise? Thanks.

 

Depends on the setup. I had a C4 automatic with a fixed yoke that kept coming loose, it would also pour oil out the rear when parked facing uphill. I eventually sealed in behind the nut with RTV and used Locktite on the threads to keep it from coming loose again.
If yours has a bolt-on yoke, once you pull the driveshaft pull the nut off and RTV behind it to prevent leaks, then tighten the nut to about 95-100ft/lbs, and it should be good to go. If it's a slip yoke, then you'll likely have to replace the output bushing that the driveshaft rides in, I'm not sure how much that costs though.