I have a 1999 Long bed Supercab, 2WD, 4.6L with 160,000 miles. I replace the trans not long ago. Recently it has developed a high speed (Closer to engine speed than tire speed.) vibration at 1750 RPM in all gears. The vibration goes away around 2500rpm. It is felt in the streering wheel, gas pedal, dash, and rear view mirror. I can get it to occur driving and at a stand still, both in and out of gear. I have tried new balancer, flywheel, and have removed the torque converter to isolate the drivetrain from the engine the best I can. Vibration is still present. The vibration is stronger when there is more load on the engine and smaller when there is less load. No engine code are present.

Does anyone have any idea what this might be? I am willing to try just about anything to save this vehicle.

 

I have seen fuel system component and/or fuel system delivery problems on larger vehicles (class 5,6,7,8) that exhibited problems like you mentioned here. For example: fuel supply hoses delaminating from the inside and momentarily sucking shut and then re-opening. Our 99 seems to have higher than normal engine vibration above 4000 RPM, but it doesen't seem to be severely annoying.