Transmission problems with 1994 F-150.?

Lately I have noticed strange sounds, vibrations, and hard shifting in my automatic 1994 F-150. This was especially evident at 55mph+. I had it checked out at the local Meineke and they found nothing wrong. The problems had abated for a while but have recently returned. Today, drive went out on my truck. Reverse, 1, 2, and 4x4 still work fine but the truck will not move under its own power in drive. What could be the problem/solution? Thanks for any help.