Strange tranny problem

I need some help. I have a '98 Expedition that has been performing flawlessly until last Thursday. I was leaving for work and backed out of the driveway put it in drive and it wouldn't move until I got the RPM upto about 3500 then it moved slowly. I got it around the block and back into the driveway and took my other car. The weekend came and I tried it again now it worked fine. Monday morning came I backed out of the driveway and it wouldn't go forward. A couple of minutes later drive worked perfectly but when I got to work suddenly reverse wouldn't work. Plently of fluid. Ran some seafoam through it and drive still works but no reverse. When it is in neutral it is still like it is in drive. Is this a linkage problem or what is the deal. Please help me out.

sorry but you need to get it to a tranny shop asap. hopefully it can be fixed by changine some solenoids and gaskets. if you keep driving it like this the clutch packs and bands might get burt out and the tranny will need a complete rebuild. good luck