Classic case of Death Wobble. Do an internet search and you'll find plenty of info. Is your suspension stock? How many miles on the truck? Over sized tires or aftermarket wheels? If your suspension is all stock, or even if it's not and your miles are getting high it could be time for a front suspension rebuild. I'd replace everything including ball joints, tie rod ends, track bar bushings, steering stabilizer, shocks, and the drag link. Good luck.