1991 f150 automatic trans. problem

Hello, I have a 1991 f150 5.0L with auto transmission with electronic overdrive. I need help figuring out what to do as it revs up when in gear but does not move. For the past while the truck would not shift gears smoothly. Sometimes the motor would rev-up between gears. The other day I was driving up an incline and the truck would not kick out of overdrive. By the time I could press the override button the engine reved up and the truck decelerated. The engine then stalled. Battery had some juice but not enough to turn the motor over. Towed it to the garage (rear wheels lifted). Garage put 7 liters of oil and said it worked during his short test drive. Went to pick it up and the truck will not move. The engine does not even drop in rpm when put in gear. Oil appears dark but no strong smell. Do I need a new transmission or should I check something else?

Any suggestion

Thanks

Charging system has to be working correctly before anything else. 7 liters???Whoa. Maybe it pumped it all out after/during their road test. Should be pretty obvious where that much fluid leaked from(Front seal area, or cooler line). Check the fluid level again. Low side of stick, engine running in park.

I am having pretty much the same problem with my 93 F150 in gear it's fin but if i shift to neutral or park it revs right up, when it shifts between gears it gives you a good kick you know it's shifted