Possibly the center stack pin/bolt is either loose or broken?

 

My 04 SCab is doing the EXACT same thing...overloads shifting to one side. As far as I know my issue did not start to occur until after I installed the 2" drop sakckles and shims.
The only difference is that mine only has the issue on the rear left side. Funny because on that left side it seemed like the spring stack pins were not as long as on the right side. I had a devil of a time getting the holes aligned on the pins.
But in truth my spring has moved to the side about as much as yours, after about 10,000 miles each time.

I have not gone to the dealer cuz I imagine they will see the 2" drop and tell me to go blow...
I'd be interested to hear what your dealer says

 

Simple solution.

Knock them back straight and then re-torque the U bolts to factory spec. It is very common for the U bolt nuts to loosen up when new.

Torque Specs are 85 ft/lbs (light duty) or 184 ft/lbs (heavy duty)

 

welp i had fun at the dealer.. they told me that it was normal.. they said they looked at 6 brand new 06 F-150's and said they all look the same.. so i of course looked for myself and confirmed it.. atleast on some.. some had the same thing happening and these were brand new so.. another entresting thing i noticed on all the 06's is that they only have 3 leafs sandwiched together where as my 05 has 4... strange... sence i saw one with a GVWR of 7200 and mine is only 7050.. i dont get it..

o well think ill try 928gt's solution thanks guys!

 

Ahemmmmmmmmm......

Keep that hammer or rubber mallet handy. They'll be out of whack again one of these days.

..... and if that bottom spring ever matches the arch of the top ones without a load on it..... then you have yourself some problems.