hey guys could anybody tell me why my truck runs great in 1st, 2nd, and 3rd gear but when it shifts into 4th, it starts shaking uncontrollably? It's an 89 with 4.0 renix (obviously) and I have brand new fuel pump, gas tank, injectors, spark plugs, tps, IAC, and CPS. I can't even get this thing to go on the highway because I'm afraid it will fall apart. the problem is ONLY IN FOURTH GEAR and I'm completely stumped.........