Hello all,
I have been trying to figure this one out for awhile now but I'm about ready to give up. I have a 99 f150 extended cab short bed. There is a 6 inch fabtech lift on "Moby" but I dont believe that has anything to do with the problem. I have researched forums here and on f150online but mostly everyone else dealing with the bounce in the rear end has the problem only during braking.

The problem:
At speeds between 44 and 47 I experience a big amount of "hop" in the rear end. When looking in the rearview mirror you can actually see the bedrails and tailgate moving up and down. This is not to be confused with the bed flexing or moving itself, but you can tell that there is a large amount of uniform movment. Further, any speeds +73 causes a vibration and shake throughout the entire truck.

What I have done so far:
-Balanced all 4 Pro-Comp Xtreme all terrains (37") - that didnt do anything
-Replaced all 4 tires with 37" Falken Wildpeak a/t and had them balanced - still no help
-Full alignment
-Replaced drivers side rear shock because the bushings were bad
-Replaced rear u-joint
-Driveshaft was balanced 2 years ago due to slight vibration at high speed but this is a totally different feel
-Rear rotors were turned, front rotors still look good, all 4 tires have new pads
-Took diff cover off to check gears, no metal shavings or anything so I guess it's good. New gear oil
-On the front -- Upper and lower ball joints replaced, inner and outer tie rod ends, idler arm, steering shock
-Full alignment

So, basically I feel that I am running out of things to replace (and money). I could have the driveshaft rebalanced but it really does not feel the same as my previous d.s. problem. I have read stories about many super duties that have a bouncing or vibration problem and they are told that it's the nature of the beast. I have a hard time swallowing that. I have driven this truck since 2006 and until about a year ago I never had any driveline issues except the driveshaft being unbalanced. Before there was not even the slightest hiccup at 90 mph so I dont understand what has changed.

The only other things I can think of are replacing the remaining 3 shocks, getting the d.s. balanced, and replacing the other u-joint near the transfer case (which seems to be in good condition and I dont expect would make a hop rather than just vibration). If anyone has any information on this issue or has dealt with a similar problem, please help!!

 

This may be a little stretch, but I had a similar problem in a Chevy 2500. I ended up replacing my leaf springs because the center pin broke and the springs them self had an unnoticable amount of movement which in turn caused the rear end to become off center, but it was enough to cause a strange hop like my brakes were locking up.

The only other thing that I could think is you could have an issue with your brake caliper itself. Worth checking.

Leaf springs arent' usually cheap. I got REALLY lucky and a buddy of mine lifted his truck and gave me his which only had 10k miles on them.

Another thing too, check the play at your pinion and output shaft bearing on the trans case. That can cause similar issues.

 

You might want to check into having your tires "road force" balanced. Had a similar issue where at like 58-59mph would get a bouncing or vibration with tires and ended up getting this done and it took care or it. It's kinda of pricey like 60-80$ around here at ford dealership. Good luck hope you figure it out I hate crap like that!

 

Interested to see what you figure out. I've been having pretty much the same problem with my truck. 2003 F-150 ex cab sb all stock with new Rancho shocks at all 4 corners.