90 Mustang Auto Transmission Problem

Hello everyone. Have a 90 Mustang 4 cyl. Auto transmission. Car has about 60,000 miles on it and runs good. Last couple of days, when in the drive position, you can feel the transmission "shift".
So checked the fluid level and was down so filled it. Went to recheck it and now it either wont go into gear or when it does go into gear it shifts hard. Called a few places and they recommend a rebuild job for like $1900. Like the car so I am trying to see if it is worth it to rebuild or replace or is there anything I am missing that perhaps I can check myself? Thanks for your time.
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