[777stickman]

I might be inclined to agree with them. Stock vs aftermarket is around a 2" difference on tire diameter with the aftermarket being higher. This alters your oem gear ratio to a higher gear and more stress on the motor and power train.

I would try this: Run the truck at hiway speeds in one lower gear or maybe two. See what happens. This would tell you if it's a driveline or engine issue.

I Think there are also devises that can disable the 6 down to 4 thing out there?

Let us know.