My friends impala won't shift ot of first its a th350 it has been rebuilt with a shift kit and different clutches. It has been inspected for not shifting out of first. Everthing good. We were thinkin maybe the detent cable not adjsuted right. Or something with the shift kit. The guy at the tranny shop said He will get one that does this every three years or so so he gave us some kind of a shift plate to put in it. but who knows?

 

On the passenger side of the transmission is a cover over the accumulator spring. If the spring is broken then it will not shift out of first.
Your going to have to look it up on how to change it cause frankly I forgot except that its a booger to get the new one in if its still in the car.
Also have you checked the vacuum module for vacuum? Sorry the detent cable doesnt have to be hooked up to shift up only to kick down.