I need help
 

have a 13 tahoe transmission went out installed a used 6l80 . From a 13 silverado. Everything seemed to be working good however after you go over 40 then come to a stop it feels like somone is rear ending you. I was wanting to know if this transmission needs to be reprogrammed ,flashed or relearned . I cannot figure it out. It also after going over 40 when I stopped and tried to go it revved up and then slammed into gear . Any help is appreciated it's driving me crazy message me at twneels@gmail.com