too much pressure and bad shocks??
My Family owned a '98 Lincoln Navigator, and the steering wheel never "shuddered" or anything of the like,, and the ride was almost always smooth,, but at 50K miles with some Michelin A/T LTX tires, the fronts were all mesed up. They were lumpy, and were wearing on the edges.
The truck ('98 Nav. )had horendous shocks on the front,, from the outside you could see the tires wiggle only slightly up and down.
The tires only had 32PSI in them. (recomended presure)
If your tires are doing the same thing,, or just wearing in the middle,, you could have a worn shock(s) and with 50PSI the tires will get larger in the middle of the tread for a millesecond while the weight of the vehicle lesens on that tires. . which would cause excessive wear in the center tread.
Something probly worn? Maybe not worn out, but just worn,, you say you are on your third set of rubber, I am on my 3rd or 4th set of rubbers on the front. I still have the original shocks,, but I have been needing shocks for a while now. My tires are all screwed up,, they have that "wave" in them,, All the little tread rubber blocks are canted to one side. (the back).
Any little mis-adjustment,, worn shocks,, too much pressure, not enough pressure, a worn bushing etc. etc. will show up quickly with the 1,200 pound Cummins riding up front. Very quickly,, even just slightly different tire pressures will make it feel like your alignment is off.
Hope this helps,,
Don't mean to "wear" you out. . no pun intended

(maybe a little pun)
MerrickNJr
P. S. My Grammer, and speeling was sub-par,, tried to fix it,, it's a little better now.
EDIT #2 LOL Look at that P. S. I'm going to leave it like that. . to bad. So I can't spell.