I have found that the 12 bolt pinion can loosen up over time. It uses a crush sleeve to set pinion preload, and I think it just crushes a little bit over time. Some times you can tighten the pinion nut back down a little which keeps the pinion from moving around so much. It won't fix the increased backlash, but it does help with the vibration. The 12 bolt is so strong that if you really wanted to fix it I have no doubt that is you installed a new crush sleeve and reset the backlash and pattern it would be like new. The bearings and R/P are big and durable, unless completely abused you can probably reuse them.