I am betting the problem is the 10 ply tires.
Have a good front end shop go over it and see what they reveal.
The 1/2 ton models were designed with ride comfort first, towing second. Mine came equipped with some pretty soft Firestone's that rode great but were completely used up by 35000 miles. I replaced those with Michelin P-rated that lasted a bit longer but the ride was a little harsher with the thicker sidewalls. I can only imagine the difference had I went 10 ply. I never went any heavier as we upgraded to 5th wheel and bought the F250.
You have already ruled out the shock issue, I am betting the rear if it is the LT model was the worst culprit on the rough ride, that was the result with my Suburban, the optional soft ride shocks had lost the first stage and they were harsh, replaced with new Monroe's and it got better.
The intermediate steering shaft recall on mine felt like something loose at the floor level, it did not make the ride harsh but did produce a clunking sound that was annoying and took a couple of grease repacks without success until GM designed a replacement shaft that cured it.
I would think that your tie-rods and ball joints should not be at the replacment stage yet with those miles unless the truck got a lot of hard use on rough roads, but a good front end shop can verify that.
