I just got to thinking a couple times last year I would shore launch. I would have to back my truck in to the water as deep as the floor boards. How easily is it to get water in the diff and damage it? Should I worry or do a lot of people do this?
I do wheel in the water and mud as well. Diffs have breather tubes and may suck in water. If you find youself in water alot, one way or another, you may want to consider and extension on the tube. Not an expensive mod to do. Just my 2cents
I would worry a lot about getting the differential in the water. Yes it is true good 4 wheel drive it should have no effect. That because the vent hose goes up 3 feet above the differential. On my truck the front and rear differential and the transfer case are vented in a dry area at about the height of the top of the radiator.
In salt water I do not even let the back tires touch the water. If I were you I would adjust the trailer and maybe even add a tonge extension.
Things you can do.
First how high is your motor off the ramp with the engine fully down?
I like the motor to be at least 4 inches because someday you will forget to raise it and you do not want it to drag.
If it is higher than 4 inches then see it there are other holes so you can lower the boat on the trailer. My trailer has 4 positions. This lets you launch with out back in as far and also put the boat lower so yout two will cause more air to go over the boat and have less wind drag.
Of course you want the support under the stringers but on V hulls moving bunks or roller toward the chimes also lowers the boat.
On my boat I can walk around the back of the truck and step up on the trailer without getting my feet wet. Only one shallow angle ramp where I have any trouble.
I did cheat when I bought the boat by having the trailer modified to my spec.
I got a frame 2 feet longer than the boat was being sold on. I also had them double the number of rollers for more hull support. Last since My boat is aluminum I had them put in a lighter spring to better match the boat weight. This actually saved me money. All that cost me $800 more back in 1980 but it has been well worth it.
I would worry more about being able to get your rear wheels off due to corrosion. The lakes here in Michigan have been real low. I had to beat mine off with a sledge. Now i coat my rear hubs with anti siez compound.
Getting your axles in the water can get water in the diff. Not to mention the brakes and bearings. How much, depends on how well your seals are setup to hold... Brakes, well, disc are a bit better with water than drums as most of the moving parts are internal and protected.
