The code should be stored in the PCM memory .I would go to a good
trusted mechanic and he can pull the code with his scan tool .It does not need to be a dealer ,unless its warranty stuff, nor a transmission shop.
The trans probably was hot ,but It should not be in overdrive except on the flat roads ,no grades ,put it in 3rd.The 4L80E is a stout trans ,but heat will
kill an automatic trans .you have the 3:73 ratio so even more reason to not
use overdrive on any grade .Pull the trans dip stick and smell the fluid ,it
will have a burnt smell if it got toasted ,and the fluid will not be nice and red
more orangish in color.No mistaking that burnt fluid smell ,even for a non mechanical person.The allison 5 speed is a extremely heavy duty transmission
,but must also be cared for as well.