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.
I had the vehicle sitting on the ground so I was not able to rotate the diff. Thank you very much for the reply I guess I will be jacking it up and removing the cover again.
If you have a multimeter that can check for resistance ( or Ohm's ), I would use that first before taking the cover back off, it would be an easy first check. All you have to do is to take the leads and make sure they are connected to the meter in the appropriate places for doing a resistance check, then unplug the sensor, and touch one meter lead to one pin of the sensor ( not the harness connector ), and the other lead to the other pin and note what reading you got. If it's "open" as in not having any connectivity then you will need to replace the sensor. Remove the sensor and check for any physical damage, hopefully it is just the sensor that has failed due to time. I think the resistance should be somewhere in the neighborhood or 900-1300 ohms. I can look up the exact number if you are going to try this first. Let me know.