I honestly dont know what the problem could be, I own 2 '88 Legends, an '89 and a '90 as well. One of the '88s transmission went, first by making a clunk sound when shifted from any gear to Drive, I found out that the clutch plates were going. The only problem at the time was that it slipped from 1st to 2nd, all of the other gears worked fined. Then after a while the clunk was gone, but it only shifted to 1st and 2nd ONLY, no 3rd, OD nor R. Needless to say that i took the engine from that car as a spare one. My '90 still shifts fine, but after being in Drive for about 20 seconds, the S3 light flashes and the speedometer doesnt work anymore. I use Dexron III since Dexron II is no longer available, plus a little bit of Lucas Transmission oil as an additive. The other '88 shifted very rough when I first got it, I replaced the transmission fluid and added the additive mentioned above. I have babyied it for a while and the shifting has improved significantly. I suggest replacing the transmission fluid for starters, since replacing the filter is only possible by the dealer(not to mention the cost). Chances are you will have to replace the transmission, these cars are known for weak transmissions and super engines. Hopefully what I've said helps you at all. Please do some more research before doing anything to your precious car and always remember that what works for me may or may not work for you.
good luck buddy!