I'm going against the grain here to say that 2" behind is no big deal. Yes it will take weight off the front axle of the truck, but not enough to be dangerous unless you've got a ridiculous unrealistic pin weight, like 10000lbs.
We have no problems hitching a 500lb tongue weight trailer to the rear receiver of the truck, 54" behind the axle. That 500lb tongue weight pulls more weight off the front wheels of the truck than a 5ver with a 4000lb pin weight 2" behind the rear axle. Not a single person here would have a moment's worry about going ahead and hitching up that 500lb tongue weight. Yet, it's a "freak out moment" to put a 5th wheel pin 2" behind the axle?
The hitch manufacturer gave you those measurements for a reason. Most likely those measurements are to avoid cross members and critical components on the truck's frame. If you can figure out where the rails will end up bolted underneath in the recommended location, you can measure 2" forward and see what's there.