My truck has a Marmon Herrington Conversion.
The truck runs three double reduction axles and has a 46,000 gvw.
One disadvantage is that it can raise the truck about one foot in height.
The tires probably get more damage offroad that from the increased wear, if there is any.
My truck is in two wheel drive on the street.
Fuel milage will suffer because these truck are geared very low. My truck runs 6.11 in the axles.
Mu truck has a straight axle mounted under the springs with blocks so that the front driveshaft will clear the oil pan.
Locking hubs are available up to and including the 12,000 lb front axle class.
Any front axle over 15,500 lb will be double reduction and require the changing of the rear axle.
http://www.marmon-herrington.com/
Tulsa truck manufacturing has a lot of photos on their website and the also do a steped frame that midigates the increased height due to the front axle.
http://www.tulsatruckmfg.com/
Over the years I have driven many trucks both on the highway and offroad and unless you are going into some really soft stuff like sand or deep mud you probably do not need AWD. The best non AWD truck that I have driven offroad was a MACK DM600 rolloff. What made this truck good offroad was the fact that it had airlockers in all the differnetials and lugged tires. I was able to get this truck stuck in spots that would not have stopped an AWD truck. Fornately I was always able to get a Dozer to tow or push me out. So my advice is if you do not go with AWD and take the truck offroad add one or two winches, pto or hydraulic.
Another thing you have to think about is ground clearance and the rear overhang.
When we go offroad we go with a group of other trucks that are in the same weight class as us.
I hope this information helps.