'90"s have a dipstick and you can chnge the fluid and filter, which I would encourage you to do. That may do the trick.

If not, the seal on thr main shaft could be on its way out. Winfred has a great write up on this in the drive train section of this forum. I did this repair to mine when it started slipping and it worked perfectly for about 35K. Then, it wouldn't shift out of 1st until it warmed up.

I changed the fluid and added a bottle of Lube Guard (red) as per Kirk the guru's suggestion. Now it works fine.

IMO, sounds like the seals are going.