Try pushing the truck forward and back in neutral. That will tell you if you've got dragging brakes, or whether there's something causing the driveshaft to bind up going forward. You could also jack up the rear wheels and put the axle housing on jack stands to see if the drivetrain works properly in forward and reverse. If it passes all those checks, you probably have a problem getting the transmission into first gear for some reason.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!