If you want to keep your current vehicle skip the 3.73's. The difference from the 3.54 would leave you disappointed. If gear change is desired, start with the 4.1's. Have you considered smaller tires? This would be a cheaper alternative than regearing. Final drive ratio is a combination of tires and gears. Is the truck a 4wd? If so, the price is double. What about a 28" or 29" tire for towing? Avoid Mud or All Terrain tread for strictly towing. Increased rolling resistance is also bad.
Are you experiencing high temps while towing? I recently added hood vents to my offroad Jeep. I think the computer made some adjustments to compensate for extra cool air. It felt like adding 15-20hp.
Be sure to compensate the speedometer. Don't go crazy with HP mods. Gas engines don't respond well to mods.