I have a 2000 4x4 durango with a 4.7L engine and 3.55 axle ratio, which has a max trailer weight of 4400lbs. I regularly tow at this upper limit (2-horse trailer and 2 horses). I want to get a bigger trailer (ha ha,what horse chick doesn't! [8D]) that would boost my loaded trailer weight to 6000-7000 lbs.

How feasible is it to change the axel ratio to 3.92?

I'm having a hard time finding 5.9L trucks with a 3.92 ratio. I've found 5.9L with a 3.55 ratio which is rated for 5600lbs. If I pulled more than that, what am I risking- blowing out my engine? or not being able to stop the trailer, fish-tailing, etc.

 

Towing much more than the rated amount puts undue stress on the drivetrain components, from the engine all the way to the axles. You're probably going to overheat the engine and/or transmission before anything else would fail, but I don't have a lot of tow experience myself. Also, towing a large trailer without brakes could create a dangerous stopping situation (greatly increased stopping distance)and you'll probably overheat your brakes, rendering them useless. Look for a D with the heavy duty towing package, it will have the proper hardware (hitch, wiring, etc), gears, and transmission/power steering coolers to help make the load a bit easier on your truck.

Oh, and I don't think it's that feasable to change the rear end from 3.5 to 3.9 gears as you'll have to upgrade the front end to 3.9 gears as well. I may be wrong about that, but I remember reading that here.

 

Yes, if change gears in the rear, you will need to change gears in the front.........if you are planning to use 4 wheel drive. It makes good sense to change them anyways. You bought a 4X4 just incase you have to use it!