94 ford has power, no torque

I have a 1994 Ford F150 4x4 with the 5.0 engine and 4 speed auto transmission w/ overdrive. It has about 98000 miles on it. It runs well and has plenty of power on flat land, but when climbing a hill or towing a trailer, it can barely make it up the hill. About a 6% grade for 2 miles. I can't decide on whether to sell it or maybe make some modifications to gain some torque. Maybe a throttle body out of an Expedetion or Mustang. Or maybe a 5-speed. A friend suggested these. Any suggestions would be greatly appreciated. Thanks.