i'm having an issue with the rear-end of my 2010 XLT SCAB wanting to bounce at moderate speeds. the truck has 75K miles on it and i've been experiencing the issue for the last 5k miles or so. at first i thought it was the tires, so i swapped out my BFGs with new Federal Xpolra's and had them balanced when they were put on -- at the same time i swapped-out my KMC XD Monsters for stock F150 Raptor rims. while the ride got a lot smoother, the bouncing didn't go away. then i decided to replace the shocks front/rear: Bilstein 5100 series. and again, while the handling and ride are drastically improved, the bed still wants to start hopping at around 45mph.

the only way i can get it to stop is to speed up or slow down. yes, with some weight in the bed it improves (250lbs+, dirt bike and 10gals gas & water, etc). but i haven't had this issue for the last 9 years/70K miles so an empty bed is not the true source.

leaf springs? bad axle? there's no clicking or grinding, in fact the truck is running great! except for the bounce.

suggestions?

 

I know you checked or did a few things but here's what I check (I know I'm repeating some of yours).
Rotate tires in case a tire became out of roundness or a belt inside shifted, shocks on all four corners, visually check rear leafs for even a cracked leaf in the pack, check drive shaft, check all connections of the rear leaf spring supports and brackets for break or crack, check the rear rubber bushings for excessive play.
If that doesn't reveal anything starting moving forward and check the whole front end. But shocks or tires are the most common causes. I'm curious what it is.

 

Are you running factory tyre pressures? (35PSI)

 

Bouncing or hopping would indicate large uncontrolled movement of compression and rebound. Not to be confused with a vibration or shimmy.

What was the equipment,specifically aftermarket installed at the 70k mile mark before the bouncing? Once a vehicle is modified it has the potential to add to the complexity in solving the symptom.
There's additional complexity possibly with doing the shocks (ride height) as this effectively changes the weight distribution on the front and rear axle and of course the tires, no two tires ride the same.

With the age of the truck I'd want to focus on bushings. Leaf spring bushings, shackle bushings, body bushings, transmission/xfercase bushings as points of inspection

Have you visually viewed the bouncing while riding in another vehicle?

What size tire is the Federal Xpolra's? What's the load class and rating?

 

The first time mine did this on a section of highway it scared the crap out of me. A few miles later it stopped. After several trips on this road with the same symptom, I noticed it only did it on this exact section of highway for about 2 miles. Finally figured out the expansion joints on the highway were spaced exactly right to create a perfect harmonic up-down movement of the truck bed at 65 mph. Crazy. Still does it on this section of highway.

 

A similar thing happened to my friend's Ram 3500 while towing a trailer with a slide-in camper in the bed, he was cursing the local highways as he'd never run into that situation in any other state before. My own truck developed a vibration between 65-70mph after I moved, and it took me a little bit to realize it was the new section of highway I traveled on and not an issue with my truck.

 

The bilsteins fixed my bounce. By chance do you have a leveling kit on truck. Read in another forum or this one the bounce started after the 3" spacer in the front was added.