99 xcab stepside. at around 40mph i can see my bed shake, bounce goin down the road. but only does it between 38- 42 mph. when slower or faster than that it doesnt do it. like theres not enough weight or something not back there. has anyone had their stepside do it. i had the shop truck when i worked at a ford dealer do the same.

 

raise the rear with a floor jack and turn the tires by hand.. could be a bent rim or a "tumor" on a tire lol.. worse case scenario the axle but highly unlikely unless it was wrecked

 

that or possibly an out of balance tire. or maybe bed bolts aren't quite tight enough and it just happens to be at about those speeds that the road vibration is right to make the bed hop around... Also maybe shock or leaf spring issues? Not sure about that, but never hurts to investigate it

 

its not a wheel or tire thing. nor bed bolts. just to say the whole rear of the truck, bed and rear part of th e frame bounces. and it not shocks. i got new shocks but it always done this.

 

My scab styleside does the same thing. I get a good little shimmy goin on about 40mph. I figured it just needed new shocks but maybe not.

The other idea I had was maybe to do with cranking up the torsion bar. When I bought mine a year ago it had already been cranked up all the way so I have no idea if its got new keys or not? (probably not and probably will turn it back down just a bit one of these days)

Are your torsion bars cranked up?

 

check the drive shaft, ive seen the newer f150's do this too.. its weird

 

what exactly on the drive shaft? like balance, dented, or u-joints? I also have a lifted grand cherokee with 32" tires. I know all about the u-joints and their weaknesses. those hum pretty bad, progressively worse, and give off quite a distinct vibe if you know what it is.

I'm pretty sure its not the u-joints on the drive shaft because like he said = its really only around 40mph give or take 1 or 2 above or below. other than that it rides just fine. when the bed is loaded up it doesnt have this issue either.

 

make a visual inspection, doubt thats the problem tho, u said its only a 40 mph+/-...

 

i'm just bummed that it may not be the shocks. i thought it made total sense till p.a. said he replaced his to no avail. oh well, its really not that bad, maybe just an empty bed truck thing. still curious if its cause of the cranked up torsion bar though?

 

my shocks, u joints are new. and my t bars are cranked all the way up. it must b something with the step side bed.