What's baffling to me is I can put my stock rims that came with the truck, and the tires are horrible, chopped up from driving with the front end worn-out ,almost bald ,put them back on and I can drive it down any rough road I choose, and it breezes through with out a hitch.
I don't know?? I've become very good friends with Big Red of Big Reds tire shopp, as we discussed my situation he said if I had aftermarket rims he would lean towards the rims being bad. But since these are Ford factory rims, same bolt pattern and all you'd think it's the tires especially since I used to run King Ranch 20's and had no issues. I wish I knew someone that had a set of factory Ford 20's that would let me use them on the same bad spot on the highway just to see what happens.
Have you considered even tho those are stock rims, they might be bad? Warped/bent something along those lines? Since it seems to happen at certain speeds, I'm wondering if it isn't a very slight deformity - Maybe dismount the tires from the rims and check the rim with a dial indicator? Use the part of the rim where the bead seats instead of the face or some other area.