Rear Diff. 12bolt

I have a 72' GMC with a 12 bolt rear diff., 3.73 gearing. I just got it three weeks ago, and when I was changing the oil I could see some scratches like on the diff. gear housing or pumpkin inside. Since that I have driven it once. And when I deccelerate, theres a whine sound to it. Then when it heats up, it sounds even worse, like its going to fall out. I need help deciding what all should be changed and replaced, and what gearing I should go to. Does it have to do with the rear diff. that I cannot start my tranny in first, b/c its a granny tranny, or is it the tranny that needs re-geared. Anyone from So Cal. Area that reccommends a diff shop?