Well, the guys have pretty much covered it. On any truck (pickup or SUV platform), there's much more than gears that go into a complete tow package, although a lower (numerically higher) rear gear does make towing easier. Brakes, cooling, a proper hitch, upgraded rear suspension/sway bars, etc. all work together to make those high tow capacity numbers SAFE. The gears just help get the load moving and maybe provide a bit more engine braking on long steep grades.
But, as far as gears alone go, if I were considering a re-gear I wouldn't be afraid of a 3.55 or even a 3.73. There's virtually no downside to those ratios, you'll never notice any negative aspect. These trucks have multi-speed overdrive transmissions and an economy minded factory ECM tune, you'll do just fine. The performance gains from a 3.55 or 3.73 will make it feel like a new truck.
Keep in mind, if you re-gear a 4WD or AWD truck you not only have to re-gear the rear end but the FRONT differential as well. The front and rear gear ratios must match or your transfer case will go kaboom.
can you clarify >>.CHANGES THE SIZE OF THE GEARS ? as in ring gear diameter ? that would be a whole axle assembly (housing included)
Changing ring and pinion gearsets is a common performance upgrade. It can be done without changing the whole axle assembly. Take out the old gears, toss in the new ones. As long as you don't go too radical, the change in ring gear diameter shouldn't matter - it'll fit.