I bought the vehicle used with the abs light on all the time. It seems to stop o

Chunks of metal found in the differential aren't really ever a good thing, but it may not be the end of the world either, depends on how big the chunks are and from what component they came from. If the ABS lamp is illuminated, it basically means that the control module has detected a fault and disabled the ABS braking, it will have no affect on you base brake system. On a 90 Bronco, and off the top of my head, I'm guessing this is only and RWAL ( Rear wheel anti-lock ) system. If this is the case, the most likely culprit is the differential speed sensor mounted on top of the rear differential. It should be a 2 wire sensor. These commonly fail electrically, but they can also fail due to physical damage ( like chunks of metal smacking into it ). Did you by any chance rotate the differential while you had the cover off and take not of the condition of the ABS speed sensors tone ring? If your metal chunks were coming from there, then you have most likely found the root cause of your ABS issue. Of course there are other possibilities, but I have found that the speed sensor in the rear differential is the issue most of the time. If you have any other questions just ask.