1994 Corolla Differential service dilemna

I posted on this site about ten days ago about a vibration/shaking my 1994 Corolla w/1.6L 3 speed AT was experiencing at high speeds. I since have taken the car to an AAMCO shop as well as a private repair shop, and after thorough inspections they both told me the differential is faulty! My car has 200miles/12 days from today left on a full 1000 mile/30 day warranty. My question is: A- how much would a differential rebuild cost?
B- How long would it take
C-What steps should I take should the Dealer believe he is not responsable for that repair?
Any advice is greatly appreciated

Go back and get the estemates from both repair facilities. Show them to your dealer. Tell your dealer to cover the cost or cancel your purchase agreement, as per your implied warranty on your bill of sale.