Could be the TCC solenoid in the trans. the 4th gear is having trouble staying in 4th while under heavy load.

 

What I'm going to do is attempt to upload a video of it tomorrow. Weird thing is it only started a few days ago. A week ago when I bought it and drove it home down the interstate it ran perfectly. Got that good luck lol

 

have fuel system presure checked because a clogged strainer will cause this crap too. it will jerk while doing this slip feeling. if not sounds like your boost valve is not functioning properly.they can hook a machine to your truck and drive with it on vehicle and it tells them everything electronics and mechanicalit may end up beeing the boost valve and possibly shift censor b thats going censors are only 18 dollars at advance auto each theres 2 and easy fix if thats the problem only have to drop the pan and take filter off then censors and boost valve are on the front of the valve body behind flywheel. most tranny shops will perform this test for free however dont get takin advantage of by wolfs trying to make money instead of helping solve the real problem