Hi everybody, I'm looking for some help diagnosing a vibration in my 89 F250 4x4...I am going to try and give as much detail as possible, so please bear with me. For starters, it has a 460, C6, BW13-56, and I believe the 10.25" axle with 3.54 gears. The truck did sit for some time prior to my purchase, but the symptoms do not point to an out of balance/ round tire. It is a supercab long bed with the 2-piece drive shaft.

When driving, the vibration is only present from 30-45mph and only then if I am pressing on the gas pedal...does not vibrate when coasting. It happens in both 2nd and 3rd gears...would probably do so in first if I pushed first out that far. Smoothes out after 45mph, and by 55 it rides smooth as a Lincoln. Tranny fluid is good, filled up to proper level and not burnt, 4x4 works just fine, and front hubs are disengaged. Vibration is from the rear of the truck....let's see, what else may be of importance....tire size is very close to stock in overall diameter, and tires are not very aggressive off-road types.

All I can think of is an unbalanced drive shaft...which sounds expensive and difficult to fix. No lift kit or anything on the truck, so pinion angle should not be an issue.

I'm stumped, thanks for any help you can give.