I bought a 2003 Chevrolet Suburban 1500 December.

I bought a 2003 Chevrolet Suburban 1500 December 2010, I love this truck, always have, but its given me some kind of trouble. I need some advice. I was having to replace my rear tires every 3 months like clock work for the year, now I'm having to replace them less than 45 days (of course I'm using used tires now instead of new for obvious reasons), and I have dealt with that, of course we are always asking mechanics and tire shops what could be causing this issue and no one has a decent answer, when the tire wears it only wears on the outer 4-5 inches of each tire completely bald to the bands, but the inner 4-5 inches is still in perfect condition like it never met concrete, the truck doesn't ride screwy, or give me a handling issue, but summer of 2011 we were going to Wisconsin for a family vacation and bought a hitch extender that added a foot or 2, and instead of the attachment coming right out straight it was inches from dragging the ground, so in other words it seems my rear end is sagging a bit, and if I step back from my truck in the driveway about 10 feet it appears that the rear tires tilt bottom inward. Like I said. It has some other issues now, but none as worrisome as the rear end. Do I replace the whole rear end? Is it a toe issue? There has to be a mechanic out there that knows. Thanks in advance.