Welcome to WF.
Not sure if you really need to do anything other than invest in perhaps some better tires for backcountry driving. Some really good All/Mud terrains can do wonders. Besides, a posi on the road is really not a great idea. Throw in a little rain or ice and you might find yourself in a ditch if you have a posi.
The D35 is fine in stock form for offroad use. If you want to go hardcore, they make some nice build kits for the D35. However, if you go that route, you are generally looking at a lift, larger tires and a better ratio.