Toyota Repair: 94 4 Runner Transmission, transmission shop, oil temp

Question
'94 4 Runner Automatic 160,000 miles, transmission slipping, eventually would not go in gear.  Towed to 1st auto shop, when they went to work on it the car was ok started going into gear, checked transmission found nothing wrong.  Within a month the same thing.  Towed to 2nd auto shop, rebuilt transmission. Two months later car starting to slip again and check A/T oil temp light is coming on. Is there away of reading the computer codes without going to a dealer?  Autozone said they could only read codes on vehicles from '96 and newer.  Could this be computer related, electrical or should I take it back to the transmission place.  

Answer
I really think you should take it back to the transmission shop and aske them why it is still doing the same thing after they rebuilt the transmission, there should be some sort of guarantee or warranty.
This is more complicated than finding out what the trouble code may be, this sounds like a transmission problem, if it is slipping, it's not a computer problem, all it does is tell the trans when to shift gears, if there is slippage there is an internal problem with the transmission.