If they dropped the rear end out of the truck to do the gearing change, they would have had to disconnect the brake lines to do so. This would mean that they would have to re-bleed the brakes, etc. I would check with them to see if they dropped the rear end, or just changed everything out while it was still in the truck.
Some ABS systems require a reset in the computer if the brakes system is disconnected or worked on. Failure to reset it can also cause the ABS system to not function properly, or fail all together. Before you tow anything, get it checked. If the light wasn't on before you went in, and is on now...something changed somewhere.