allison stuck in 3 gear
 

driving truck and shift into 3rd and wouldn't go higher or low and no reverse
shut truck off and let it sit for 10-15 minute started back up and now has all gears again
would it be the computer on the side of the transmission

That is possible 6.2Lturbo but it sounds more like a failing transmission. Were you WOT when it happened?

I talked to some local shops and they said it's the computer on the transmission I have now replaced it and still locks the transmission in a gear shut truck off sit for 30 seconds start it back up and everything works again

I had the same issue it was just the sensor on the side of the transmission I replaced it for $140 and she's running great again !

3rd gear is the default gear the transmission goes into when in limp mode. There are many reasons for the transmission going into limp mode. What was it/you doing just before it happened?

Reading the transmission code could shed some light on the cause. Unfortunately most code readers won't read transmission codes. You may have to take it to the dealer or allison shop to find out