Mitsubishi Repair: Automatic Transmission Problem, automatic transmission problem, mitsubishi galant

Question
I have a 1991 Mitsubishi Galant. I recently took in in for repair of a transmission leak. They replaced a rubber seal where the axal goes into the transmission. Two days later it stays in third(maybe fourth) gear all the time. When I try to change gears manually using the shifter, there is no reaction. Reverse works fine, it just stays in the higher gear without shifting down.
The shop thinks that it might be the cars computer, but I am skeptical due to the leak repair just prior to the problem.
I welcome any ideas.
Thank you

Answer
Kevin: Got an AutoZone close by? They would code your car for free and that would rule out sensor or computer problems. Are you sure the shop got all the bolts tight? Have your checked the fluid? Believe it or not I have heard of shops forgetting to put fluid back into cars. My manual talks about missing lock pin or interlock pin missing. If the shop replaced the seal are you sure they put it in right or could be your are loosing fluid out of that seal. The fact that the car worked for two days and now is not, I would check that fluid. I think I would ask the shop to put the car up, and check their work to make sure the seal was put in right and is doing the job that it should be doing. With no charge to you. If your car has the right amount of fluid and that seal is holding, then I would say it could be the computer. But at an AutoZone could have your car coded for free and you would know if the computer is messing up. If the computer is not the problem then I would say the shop did not do something right. Hope this helps and good luck!