I have a 91 gt also with an AOD(not anymore) car had 130,000 miles on trans. Pull that AOD out and put in a stick. Don't put alot of money back into that AOD. Did my T-5 change over and overhauling engine at same time.
First thing to check is the fluid level, also what does the fluid look/smell like. After that you could try checking the TV cable that hooks to the throttle body to make sure it is still connected, although even it wasn't hooked up the trans should still pull.
Anything other that those two things I would have a Trans shop you trust check it out for you and give you a diagnosis. There are several companies that make build good replacement AODs . PA has one in the 1300.00 range and even Lentech now offers an AOD that is a little more beefed up than a stock rebuild for around that same price.
Check the TV cable...chances are it is disconnected at the throttle linkage. There is a nylon grommet in the linkage that is pretty crappy...it can crack and fall out. DO NOT DRIVE THE CAR IN THIS CONDITION. If the trans isn't already shot, it will be if you keep driving it like this.