I'm looking for some of that seasoned experience I've seen before. I'm addressing a problem I posted about 6 months ago that is unchanged. I've got an '04 SC 150 (4.6L) that I've had vibration issues with since day one. I've had the tires replaced time and again, road force balanced, the counter-balance placed on the drive shaft, the drive shaft itself replaced, the ring and pinion replaced and the dampening system put on the truck. None of it did a thing for me. I've had two guys recently tell me that my problem is a design issue in the two-piece drive shaft and its offset. Both suggest that the harmonic imbalance at certain speeds, in my case, also approx 50 mph, is the problem. Both suggest that the problem can be fixed by buying an aftermarket driveshaft, one piece. I've had it with the problem and am willing to try it, provided it's valid and worth the investment. Any insight from anyone out there regarding this issue is appreciated.

 

I have the same problem and like you thought it was a bad tire, or out of balance, or misalignment. But yesterday I flipped my rear axle and it lowered the truck so much I had to have a new driveshaft with a carrier bearing and slip joint made. And while pulling the old one I found the the output shaft on the transmission is very loose. I'm guessing the tail needs replaced or maybe just a bearing...


But either way, if you get under your truck and grab the front yolk and you can move any more than maybe a 1/16 of an inch, then you have the same problem. Mine vibrated right at 60-70 MPH. I'm trying to find the parts to fix it now.

 

too bad you can't remove the driveshaft and get the truck going at your target vibration speed, that would prove it for ya... but you'd need a motorhome to flat tow it that fast with control!