Pinion angle and how much does a stock suspension move?

[Drew Pojedinec]

So get some clamps and see what happens.  If nothing else it'll rule that out.
My view tho is, on constant accel or decel the springs shouldn't be wrapping/unwrapping unless the throttle is being pressed/released.

Have you jacked up the truck in the shop and run it hard looking for the vibration?

[cjshaker]

Perfect scenario for using a GoPro or some other video cam to watch the suspension.
With blocks and almost any type of spring, there will be a lot of pinion angle movement, even under mild/normal driving. That doesn't necessarily mean the springs are bad, just soft for ride quality.  Just braking alone will cause enough resistance to the rear to cause it to 'dive'  and that can allow the springs to oscillate in an elliptical motion and cause vibrations. Been there, done that. Weaker springs are nice for a good ride but the wrap-up has to be controlled.

[chilly460]

Just piling on with what's been said.  The lift block acts as a lever and really moves the pinion around on trucks, as I'm sure you've guessed.  I had my 390 in a '76 F250, very heavy duty springs, and i had pinion issues.  I put new springs in it with no block and the pinion walked much less.  It's amazing how much they'll rotate even at light cruise.  I know your truck isn't that heavy, but it's heavy enough and with somewhat large tires, it's using a fair bit of torque even at "cruise" compared to a car. 

[KMcCullah]

It seems like every big gain in power that I've made has caused me to have to do something with the rear end in my truck. Adding a CV joint on the T-case end of the driveline fixed my vibrations. Very similar to what you're describing, Ross.  I think your truck being a lifted shorty and having much more power than FoMoCo intended would be a good candidate for a CV joint on the rear driveline.

I've always liked a little axle wrap personally. Squatting on hard acceleration helps with weight transfer. With your lift and tall tires your getting too much of a good thing I think. If your truck was a 2wd, a set of slappers would be great. I ultimately ended up building a set of long ladder bars. But they make the truck ride like a lumber wagon though. If banding the leaves together doesn't get your wrap issue under control, I'd consider a buying a simple 3-point link from your local Tractor Supply or AG dealer and making a few brackets. I think this would be better than a set of ladder bars. Your ride wouldn't suffer as much.

https://www.google.com/search?q=rear+axle+wrap&biw=1600&bih=805&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi6q_2m0LrQAhVB2mMKHUXWC0cQ_AUICCgD#imgrc=D1go9NEpkZ9mGM%3A