Does your trans work ok now? The whine could be a number of things mainly being bearings inside the trans or the torque converter. Again I dont know what you mean by 4T60HD because there isnt one by that name, there was a 4T60E-HD in 1996 only before the 4T65E-HD went into production. As far as the differential there is no really good option for a limited slip type setup to have two wheels spin all the time. EP or Engineered Performance will modify stock differentials and is probably the only option I would seek, but I am still not real crazy about them as I have seen and read about too many failures from modified stock type differentials, but theres seem to be the most reliable. However, you have to send them your diff and it costs around $600 and there is no warranty of any kind on it. I had to wait over 3 weeks for one that I sent in for a customer so if you really want to use this option you need to plan ahead. Rebuilding the trans to handle your power is no big deal, I have built them before though its been a number of years since I have done much with them. Having the whole car would be the best way to do it so I could get the trans better dialed in to the car, but I need to make WOT runs and wont do this unless the engine is 100% sound for this kind of driving. If you dont drive the car on the street every day than a limited slip differential may be ok for you since you dont put many miles on it.