transmission problem 94 Ford Explorer started running terrib

We have a 94 Ford Explorer and it has started running terribly. It will go into reverse just fine, but it just revs up when you put into drive and then it kicks in. What could be the problem? It is so messed up! We back out of the driveway fine and then have to roll about 2 houses before it will kick into gear. Once it is running, it runs just fine until you put it into another gear and then go back into drive, then it starts all over again. BTW, it is an automatic transmission.
Thanks for any tips!! We know NOTHING about vehicles so this is scary us that it is going to be costly. We have had the transmission replaced about 5 years ago and we had the engine redone about 2-3 years ago. It is past due for a fluid and filter change. Get this done ASAP and see if that helps. Also, I would be willing to bet you have an A4ld tranny in your Ford, but it dont matter what kind of tranny you have, I would also recommend replace the vacuum modulator located on the side of your transmission. Goodluck hope this helps. Thanks for the tip! We had the fluid and filter replaced today, but they didn't say anything to us about a vacuum modulator. (How expensive of those?) Should we take it back and ask them about it?
The guy told us that it may work but he wouldn't guarantee anything. He also said that it may start to run better in a bit too. He didn't mention this to us, but somebody mentioned that it could be that not enough pressure is building up behind the seal. Does this mean anything to you? Doesn't me. It did seem to run a bit better. it still has to sit in gear a bit before starting, but it was a shorter time to kick in.
Thanks again and I look forward to hearing back from you.