Why can't I tow my boat anymore?

[Maestrbob]

I pulled my 20' Edgewater boat and tandem axle aluminum Wesco trailer for years with my 2000 Tahoe. After 254k miles the transmission finally gave in. I replaced it with a 2007 Tahoe LTZ, but I have towed three times and burned up the rear end all three times. The first two times were supposedly due to blown pinion gear and seal. The third time was due to blown axle seals. Chevy keeps rebuilding the rear end instead of finding the cause. Sounds like a pressure issue, but only is obvious when towing the boat, which weighs only 5000 lbs. Have Tahoes gone the way of the do-do? Any insight is appreciated.