[hatzie]

I've done some more reading about the ABS computer and gear swaps.
It will definitely upset the ABS computer and the Tech 2 can't fix it.

You need a pinion reluctor ring that matches the new gear ratio so that the two front and single rear ABS speed signals match.

Not enough thought went into the rear axle ABS sensor system design. The front pickups are in the wheel bearings. They're the same regardless of the gear ratio in the front axle and they are the same on both sides. One part multiplied over millions of trucks.
With the pickups on the pinion they have to be specific to a gear ratio set. If the engineering guys had thought about reducing differing parts they would've put the pickup ring between the ring gear and the carrier. This would be on the output side of the gears so the signal would match the wheel speed no matter what gears are in the axle.

A tuner (your software & hardware or a shop) should be able to set the axle ratio and tire sizes in the PCM and a Tech 2 or MDI with GDS or Tech2WIN can set the tire size in the ABS computer.