I had a 1999 with the 47RE, and 3.55 gears. It towed a 10,000 pound 5er, and on occasion up to about 14,500 pounds in tandem. In order to run above 60 MPH, even with just the 5er, it had to run in 3rd.
Going up the passes in Colorado, you really had to keep the RPMs up, or it would run very hot.
Traded that in on an 03 HO engine with the Nv5600 manual. GREAT truck. I could tow 17,000 pounds up the Colorado passes, never going slower than 50 MPH (once I added the Smarty JR). I really like my current truck, but that 03 was awesome.