After running 35's with just a front spacer for almost two years I decided to give the tires a little breathing room. So I had a 6" Rough Country lift put on my '05 Ford F-150 4WD King Ranch when I was due for a new set of tires. The kit has been installed for about 3 weeks now and I love the look of it. However, the ride quality of the rear is terrible. It is bouncy on normal roads and when you hit even a small bump it develops a shimmy in the rear that is border line violent. When my 3 year old son is in the backseat he is getting bounced all around and can not talk without his voice chattering to where you cant understand a word he says. Has anyone else ever had this problem???

Here are the specs....I went form a 35" Nitto Mud Grappler with a 3" spacer up front. Other than the overpowering roaring from the tires I had no complaints of ride quality other than at low speeds as a result of the tread pattern of the tires. Now with the 6" Rough Country lift I switched to a 35" Toyo Open Country M/T which cured the tire noise but the ride quality has dropped. I used the rear shocks that came with the lift kit and used a KYB Excel strut in the front.

 

Maybe swap to a better rear shock. Even though I have a different style truck, I've got the RC 9000's on mine and I have the same issues sometimes at higher speeds just not as violent as you described.