It shouldn't be too hard to do anything to the rear end. There is no reason that you can't have 4.10's either. The mileage between your current gears and them may be noticeable but probably isn't that great between 4.10 and 3.55 and especially 3.73's but the pulling most definitely IS. Differential swaps are normally not hard at all but as previously stated you need the suspension to support the weight. Adding a leaf is easy as well if you are a little mechanically inclined. You said earlier that you pulled the 18' boat around town and not on the highway. Is that what you plan on doing with the larger boat?
Stopping can be an issue in many conditions. For example, in ice and snow if you drive like a moron and haul butt around and expect to stop on a dime then you will be in for a rude awakening. Same thing goes for towing. Solution to the problem---SLOW DOWN and BE CAREFUL. If you are in town where you have the luxury of driving slower and giving yourself a nice cushion in front then problems decrease significantly. And the inertia driven brakes don't hurt at all. You probably don't have an F150 or 250 because you don't want one or don't have the means.
That ranger is capable of pulling every inch of a 21' boat if rigged properly. It is a truck for goodness sake. You will be surprised what it can do if you just try but be smart about testing its limits. Don't hook up to a mobile home and run 80 on the interstate your first time out.
Seems like you are doing your research and have a good idea of what you need to do. Just be informed about your decisions.
For the doubters: A guy in Delware took his 2wd Ranger and converted it to a 7.3L Powerstroke 4x4 dually Ranger. That's an extreme case of one guy doing something incredible and EXPENSIVE but that's all it takes is one to prove something can happen. Now does JefFZ6 wanting to tow a bigger boat seem so outrageous?

Either way, good luck. And sorry to all for being so longwinded. I normally live in the Powerstroke forums but browse here because I LOVE Rangers. It's is kind of sad to see that the guys here often shoot down great ideas instead of encouraging really cool prospective projects.