1-2 gear stall/backfire
 

Hello all,new member here.:).
91 gmc sierra 350,4x4,ext cab.
Alright, I seem to have a hesitation/backfire into throttle body going from 1st to second gear.It only happens once,then will not be an issue until the next time I shut the truck off,start it and go from 1-2.I can start the truck and let it idle for awhile or even rev it a bit and it wont trigger it.its not until I put it in gear and go from 1-2.
The engine has about 70k on it.I have replaced the fuel pump,f filter,f lines,rebuilt throttle body,new injecters,had fuel pressure tested.
I have also replaced:idle air,knock sensor,throttle position sensor,o2,egr,all vacum lines,tranny rebuilt,computer,distr.plugs,wires,and others.
The only code ill get is 32(egr)and have replaced it a few times.( I think the backfiring is causing the egr to blow out)so until I can get this issue resolved I will have to deal with that.
I know this is a long post but i really need help!any suggestions will be great.
I have done everything I know to do. I even took it to 2 different mechs,they could not help.