It could be a number of things. By putting a different diameter wheel and tire combination on the car you are affecting the "harmony" of all the rotating components from the engine down to the brake rotors. You see, all rotating assemblies have a speed at which they will run rough no matter how well balanced they are. It is called the critical speed. The increase in weight from your bigger wheels and tires coupled with a reduction in the highway running speed of the engine, transmission, and drive shaft could have ganged up on you to created the issue. If you changed your rear end gear to re-gain the RPMs you lost at a given speed that could be another issue. Going to another different gear may erase your problem.
But with the above mentioned, I have a prime suspect that no one has mentioned yet. . . .
You used the stock brakes yes?
Think about it. . . your wheels are heavier, which means there is more for your brakes to slow down. This could be significant since you've put more mass further away from the center of the hub and the relationship describing the inertia of a given mass is cubed with relation to its diameter. There are too many unknowns here for me to quantify how much more inertia your brakes are working against but I guarantee you for a given vehicle speed its more. The result is hotter running brakes and, likely, warped disks.
But wait its worse. . . . Your wheels are bigger around, but your brakes stayed the same. As a result your brakes have less mechanical advantage to stop the car AND reduced cooling. Even if the inertia of the wheel were the same you still have to stop the same size car but with less mechanical advantage meaning you are applying more force to the disks to stop the car at a given speed. The result is hotter and, likely, warped disks.
