All the links I look at for centre diff lock in high range say it also lights up the ABS light. But they also seem to be installing the switch in the wrong place, it should be where the hazard light switch was originally, as the cables are then the correct length, and the hazard light switch is spaced further away to be found by touch without having to look at the switches.
Im not sure if the abs is actually switched off or just indicating that it won't work correctly. It makes sense that it wouldn't as if the front whèels were to both lock, it could potentially lock the propshaft, preventing the speedo drive sending a speed signal, and cause the rear axle to lock, preventing the rear abs sensors making a reading, then the abs would think the truck was not moving, and never unlock the brakes, causing a huge skid and potential roll over.
The ABS on my 80 is only one step away from very crude, I am not sure if there are different ABS systems on J80's, it only has a single line for the rear brakes, however it does have a sensor on each wheel instead of just one in the diff. If it detected single rear wheel lock it could only release the brakes on both the rear wheels, I think the rear brakes are probably too small to lock the rear wheels in most situations anyway, the only situation would be if the centre diff was locked and the front wheels were locked.