You are lucky. Go on a Chevy board and do searches on 4L60's and see what you come up with. www.fullsizechevy.com, www.gmfullsize.com. Many guys that do any serious towing swap in the 4L80 which is stronger but robs power and mpg. And at a minimum they try to "help out" the 4L60 with tranny coolers shift kits and servos. I used to own a 98 Tahoe, so I'm speaking from experience as well. Almost everyone I know, and I mean everyone has had tranny issues with the 4L60 in their fullsize trucks. Some at 30,000 miles.
Same tranny used in a 3000# vehicle as a 6000# vehicle about says it all. 
Lets not forget the puny 8.5" diff, same situation.
BTW, have you changed your intake manifold gaskets yet? If not you should.
Gaskets were done 4 years ago, right after I bought it. The tranny is still doing good. I don't trust it for much longer though. The 10 bolt does suck ass. I'm waiting for the posi to blow any day now. I need to get an Auburn or atleast a new open diff in it.
Besides a dist,fuel pump, and radiator, it's been good to me.
