We experienced the same thing: a "big" trailer is tough to tow for the L6 motor and 3.42's. I was just in a local junkyard and found an L6 Envoy 4x4 with 4.10 gears, so I took out the rear end. It was a 2002. I wanted to switch rears with my 3.42 rear RWD TrailBlazer, 2008. I later realized the older 2002 rear doesn't have holes and sensors for the ABS near the ends of the axles. I'm also in a dilema like it: try to find a shop that will swap the used gear, or pay for new gear and reflash (expected to be about $1,000+ at a GM dealer), or attempt to DIY the gear swap, or try drilling the old rear, etc. etc. I think the gears may help but it may still be relatively wimpy, especially compared to a 6.0L or 6.2L if you find a Tahoe with a bigger motor. Someone reported it being night and day: towing a camping trailer with a Tahoe 5.3L (white knuckles) vs. a new Silverado 6.2L w/ 3.73's Max Tow package (power and control to spare). Good luck!
Rob in AZ