1994 Isuzu Trooper Won't shift.

My husband has a 1994 isuzu trooper. After hauling a large load, he strated having transmission trouble. It will shift into reverse and drive, but will not come out of first gear. He replaced the transmission with a good used one, he replaced the torque converter, speed sensor relay, speed sensor in rear differential, tcm, and it is doing the exact same thing. He found a relay that says lockup off (upshift). We don't really know what that is, but we ordered another one. Any suggestions. We really need this vehicle.

Thanks.