Sounds like it could be a burned-up band, since you state that it would not stay in gear. Also, the first thing you mentioned was a whistling sound and then it was slow to engage reverse. Whistling sounds are normally associated to the pump, just before they go out.
If I had to take a wild stab at it, which i am, I would say that your pump started pumping flaky, which would have slowed down the reaction time of the servo piston, which in turn caused your bands to slip.
You will most likely have to get it to a shop to find out for sure what happened, but that is a guess. Since you have a 98, it just might be worth rebuilding. If it is the pump, they are not all that exspensive, but they aren't cheap either. If the Bands were affected, you would be look at a total overhaul. That could get pretty pricey!!
So like I said, have a tranny shop look at it. Most tranny shops will take a look at it for free. If they cannot find the problem without tearing into the tranny, then they will start chargeing fee's.
Good luck!