You have probably already checked these things. But, I will run some of them down anyway.
Alignment
Air pressure (especially the inners)
While it is on the jacks, check the wheel bearings for easy movement and check for play.
Parking brake released fully
Clutch/es slipping, torque converter?
If all of that checks good and no heavy drag is noted it could be driveshaft alignment or binding, bearing wear in differential causing deflection, loose or worn shackles causing differential to rotate under load.
If you get a little speed up and shift into neutral does the truck continue to roll forward easily? If so, you might want to begin looking at the engine to see if it is losing power.
Those are a few of the basics that sprung to mind. I hope it will help. If nothing else maybe give you a few more ideas.