If you're not willing to upgrade to a 2500 Suburban, I'd get LT tires and some air bags for the rear coils. I use Amsoil Severe Gear® 75W-110 in my rear differential. Used it in my 1996 wagon which towed with an overloaded rear axle for many years and 8-9000 miles of towing. I have it in my Suburban now as well. You could look into an aluminum rear differential cover with cooling fins. Aluminum transfers heat better and the fins will help as well.
A vehicle upgrade is best, but if you want to keep this one going another year or two, I'd do what I've mentioned above.
