I have a duramax 6.6 and once I drive for about a mile the ABS light pops on and the truck down shifts, bringing my speed down to about 55 mph. I have been told something about slack in the transfer case and then he change to something else not sure of this guy.
ABS light pops on and power loss the truck will run 50 mph on 2002 GMC Sierra 3500
Before replacing any parts on your 2002 GMC Sierra 3500 I would recommend to have the fault codes stored in the ABS controller read and diagnosed. That should lead to the cause of your ABS light turning on.