Had the "mysterious" 35 to 50 mph severe vibration (to the point of rear wheel-hop) ever since I bought the truck. Replaced shocks, replaced rearend lube, tires balanced twice, had driveshaft totally rebuilt and balanced, and the vibration was still there. One day, took a load of cast iron (1,600 lbs) to the scrapyard and had the load removed with the magnet crane. The crane lifted all the cast iron in one grab, even lifted the bed of the truck slightly when unloading. Driving home afterwards, I noticed the rearend vibration was gone. Crazy but true. But after three days the vibration has returned.

I welcome anyone's thoughts.

 

Check the condition of your leaf spring bushings...or possibly a broken leaf on one of the spring packs. Are your springs sagging pretty good? Is it lifted/lowered at all or just stock height?
Any of these might cause the pinion angle of the rear axle to change...which can cause that type of vibration if it gets misaligned from the tranny/transfercase output shaft.

Lifting all the weight off of the springs might've let things slip back into place where they are supposed to be...but they eventually settled back down where things are out of alignment again. Try lifting the rear of the truck again...pull all the weight off the springs...then let it settle back down. You might try doing a bumper height measurement before/after doing this too.

 

The truck is at stock height. Left to right, the rear seems level but the factory bumper height I don't know. The lower spring in the pack is not aligned with the rest. The truck is 10 years old and previous owner did alot of towing but I really don't see any spring sag. Someone told me that a broken trans mount or even a broken engine mount could be the problem. It seems to vibrate shifting from 2nd to 3rd and 3rd to OD. When I have alot of throttle pressure on the gas, the vibration is hardly noticable.
But thanks, I'll check the rear bushings and leaf springs.

 

Yep, a tranny or engine mount could also cause drivetrain misalignment...or let things rub against where they're not supposed to. That'd be another ting to check out while you're crawling around underneath.

'98 is fairly old, and especially if it was a tow rig I'd bet that the springs are sagging a bit. The front torsion bars (assuming your a 4wd) or the front coils (2wd) will also have settled a bit so a front-to-rear height difference usually isn't all that noticeable. Just regular spring sag probably won't throw things out enough to cause a bad vibration, I'd suspect a fiddled out bushing or broken leaf first.

That overload leaf is a little crooked on pretty much every 97-03 truck I've been around. Usually nothing to worry about.

 

Binder, thanks for your suggestions. I'll be checking into new rear springs and bushings soon. Do you have any experiences with air-ride bags added to the rear leaf suspension? Air shocks were a thought but my past experience with them made the ride quite stiff.

 

Hi There,

I have a 2003 King Ranch SCREW and had the same symptoms as yours for months. I replaced u-joints, wheel bearings, new tires and balanced the drive shaft without success. I just replaced all the brakes (rotors and pads front and rear) and then I noticed the e-brake shoes. The shoes looked good however under closer inspection I found the shoe material was separated from the steel liner at both ends. I guess the glue let go. I believe as I was driving the shoes would float and rub the inside of the rear rotors and cause a vibration. I replaced the e-shoes and have not had any vibration for 2 weeks now.
May not be the same issue but it might be worth looking into.
Good Luck

 

My 98 has rear drums and I do suspect they might be tight. When I back out of the driveway, I hear them squeek a bit.