you've got a couple of choices ...
It'll probably need to be rebuilt. They are not inexpensive to do; a less expensive alternative is to put in Chryslers 3 speed auto (the cars standard auto tranny for fleets).
BUT, the Ultradrive does drive really well, and changes the car. Therefore, if you rebuild, drive with the tranny in 3 around town, avoiding overdrive. These tranys constantly shift in and out of 4th/OD around town, overheating them. Using third eliminates all of the problems.