I posted this on GMTN, but having issues logging in since the move to a new server. Thought you guys may be able to figure this out.
Before switching to 4.56s, everything was fine. Swapped 3.73s for 4.56s (35x12.50s) and started having ABS issues. I could drive for a while then the ABS and BRAKE lights would come on, along w/ the annoying chimes. When slowing down, once I was to walking speed, the ABS would kick in and pulse. This happened after 4.56s AND the shop pulled the engine for some other work (5.3L). I assumed it was due to the rear sensor expecting 3.73s and the EBCM not liking 4.56s and 35s. Superchips has the Flashpaq that allows gear and tire size changes in their programmer for 2003-2004 5.3Ls so I got one and did the reprogram. Semi-success. Now, the light doesn't come on, but the ABS kicks in more frequently unless I brake aggressively. All codes have been cleared and no new codes thrown.
The only clues I have now are:
1) Codes before Flashpaq were Left Front Wheel Sensor and EBCM. Wheel sensor replaced (cheap replacement version).
2) ABS only kicks in when almost stopped - such as easing up to a parking space, etc.
3) Using GPS and DashCommand for verification, DashCommand is 3 mph fast @ 60.
4) Engine was removed during gear swap - no visibly damaged wiring.
5) No unusual noises before ABS engage.