have you had the ujoints in the front end checked? i would check all of them(driveshaft and axle joints) if thats not it then i would try and borrow a set of stock front tires and rims from someone to see if its the tires and rims... maybe you have a slightly bent rim?

 

Dealer changed 1 shock, I don't know but I always thought shocks should be done in pairs or all four. i don't know if that would make it shake but it's worth a try. 265/19.5 is a pretty heavy tire for a pickup, what ply are they