I have a Jeep Liberty with a 2.8 L turbo diesel and 4000 lbs will not even slow it down. In fact is is usually quite happy in overdrive with no downshifting, cool transmission temps and gets about 18 mpg at 65-75 mph.My wife has a Jeep Liberty and it has a factory tow option that allows it to tow 5,000 pounds max. It is a workout for it to tow our Outrage. It also waddles and wags and lacks braking power on steep grades...
...Jeep has a new diesel rig available in the Grande Cherokee that is very strong and gets good mileage too. Many full size 1/2 ton pickups can tow over 9,000 pounds as long as GVW stays within capacity. I would think routine towing long distance of boats in the 6,000 pound range and higer might justify a 3/4 ton vehicle. Just for sake of discussion and reference to the vehicles I list above, our boat weighs about 4,000 pounds fueled and loaded including trailer etc.
I encountered a problem with wagging once at speeds over about 65 mph, but it was with a crappy trailer with tandem axles set up where the tongue was way too low. One potential issue common to other SUV's is that the rear mounted spare tire can interfere with having the hitch high enough on some trailer designs and will need to be moved inside the vehicle or trailer.
The only serious limitation in mountainous terrain my Liberty has had has been cooling - I had one trip with a trailer loaded close to 5000 lbs with about 50 square feet of flat plate area where I had to slow down to about 45 mph when climbing grades that were both steep (8-10%) and long (over 2.0 miles) to avoid running the water temp out of the normal limts. For severe conditions like that it could use a little more radiator but it is not lacking in power at all and makes both the 4.0L Cherokee and the Ford F150 I used to own look really anemic.
That underscores one thing no one has mentioned yet - that cooling is critical in a tow vehicle as the more weight and drag you are towing up hills, the more power you will need and the more cooling demand you will have.
I have not had braking issues with my Liberty, but then I have been towing in mountainous terrain long enough to know that with a 4000-5000 lb trailer the trailer should have brakes of it's own. Surge brakes are usually a pretty cheap option ($800-$1000) that is worth every penny if you tow on steep grades.
Personally, I think if you are looking for an economical yet capable vehicle to tow a boat, the turbo diesel Liberty is hard to beat.
I don't think the same thing applies to the 3.7 L gas version. I have not towed much with a 3.7L gas Liberty, but my general observation driving both the diesel and gas versions is that the 3.7L version is pretty gutless in comparision - in the same class performance class as my 4.0L Cherokee.