Blitz & Nukeiridium may be correct.
Check your fluids. I had a transmission in an older Chevy go bad with the same issues your truck has. The torque converter didn't get enough fluid and it wouldn't send the power from the engine to the rear end. I couldn't go over 45Mph either. I was pissed... and kept revving the crap outta the poor engine. Stupid me. Could have caused more damage in my fury.

2002 4dr Kia Sportage 4X4
Tires:235/75/15, Wheels: Black 15x8 Steel ProComp 2.5" B.S., Hubs:Warn Manual, Lift:3" UPYOURKIA, Trailer Hitch, Cargo Rack, Clinometer.