1995 Chevy Truck transmission sparatic shifting

I recently purchased a 1995 k1500 4x4 with the 4.3 auto. The truck had the orignal motor that runs excellent doesnt burn oil or leak a drop. My problem just started with the tranny would suddenly stop shifting ang stay in 2 or 3. When this happens the truck wont shift up or down even if you try manually. Sometimes if its stuck in 3 and go 2 second it will go to 2 and wont go back. If I stop the truck and put it back in park and shut it off for a minute and restart it will be fine again till I have to slowdown it get hung up in gear. The tranny was replaced at 180000. It seems to me to be a electrical issue thats not telling the tranny to shift as there will be days its fine. Do you think it could be the transmission shift module?Please help