His bio says he put 35inch tires on. That would look tacky on a 2wd model, but I think I know where your going with this Rich. If he only changed out the rear gear, then the ABS is seeing a gear ratio mismatch between the front and rear causing a ABS light.
If so, then its highly suggested that the matching ratio be put into the front diff also. Otherwise, severe drivetrain damage can occur.