I've got a '02 E350 with a Quigley 4x4 conversion & Turtle Top VanTerra shuttle body conversion. Though I don't think that either have anything to do with this. It has a V8. At about 40 to 50 mph, under slight throttle (speed maintenance, not accelleration) it has a _________. I don't know if it is a vibration, slip, chug or miss. I would call it a chug. I can't tell if it is coming from the engine, trans or driveline. When I put my foot into it, it goes away. I was driving a friend's F350 of similar year the other day, and I noticed that it did the exact same thing. When I first bought this unit, it had a very similar feeling slip when I accellerated around corners. I guess the rear end uses a special anti-slip additive, and once changed, the slip from the rear end was gone. The 40 mph chug has been going on for a couple of years, and hasn't gotten better or worse in that time.

Anyone have any idea what I'm dealing with? I'd like to figure it out. I kind of figure that if it is caused by worn driveline parts, that it would have gotten worse with time, and it hasn't.

Oh, and BTW, I do a lot of towing with this rig, and I do notice it more under load.