did you tighten everything down while it was in the air or did you let it sit on the ground to tighten everything?

shackles need to be tightened while under load. if you tighten it all down while its hanging in the air, it wont be able to move as designed and ride very rough.

 

I tightened it while it was on the ground.

 

I wonder if the axle is centered. It takes a lot of lift to release the load when you're replacing the springs. Could it be offset to one side? Possibly measure the axle or follow behind on the road in another vehicle.

 

Just took some measurements, axle seems to be centered. Not sure what the problem is but im going to leave it alone. I can live with the vibration the truck is on its way out anyway.

 

Your pinion angle is probably off and should have been change when going to different springs.

 

I never knew this. I'm planning on upgrading to the same springs shortly. Guess I better have mine professionally done.

 

This could be plausible^^^^

I've swapped new springs on my bronco without this issue, but I believe it was because I had a double Cardan u joint on the driveshaft at the transfer case.. These joints tend to "self adjust" themselves.
Since the F-150 doesn't have a double Cardan u joint set up, shimming the leafs on the perch could be a good way to fix it cheap.. Shims will restore restore the driveshaft to factory spec and stop the u joints from binding.
Are those leafs factory replacements? Are you using the factory blocks?

Try taking some pics of the blocks, perches, driveshaft (u joints) and springs... We may be able to see something obvious.

 

Ok so the pinion angle seems to have been the problem. Instead of shimming the leafs i just took out the blocks. The rear end was too high anyway and i was going to do a leveling kit on the front to make it look better. Now i dont need to and the shudder is gone. Even more, the shudder i use to have around 35-45mph going uphill is gone now too. Thank you to everyone for helping out.

 

Sorry I couldn't have been more helpful but I'm glad you had it fixed. Guess I'll definitely have my professionally done when I upgrade at a later date. Must look awesome though, and definitely be able to haul some heavy weight. Kind of sad that the crew cab models like mine are only rated at 1600lbs payload.