The reason is simple. The FX is a relatively short vehicle, and putting weight in the rear will make the front to be raised. If it is only RWD the vehicle maneuverability may be compromised, hence the reason for AWD to make sure the vehicle stays in track.
Regarding the difference between 2k lbs vs. 3.5k lbs this has to do to the fact that the 6 cylinder engine, with the overall configuration of the vehicle may put the engine at high stress is towing more than the recommended weight, and hence the 8 cylinder is more powerful the vehicle can deal with more weight as the engine will not be as stressed as with the 6 cylinder.
Would I tow anything greater than 2k lbs even on the FX45? No, I would not. Why - the vehicle is a unibody, if it was a body on frame like the older Pathfinder, the Xterra or the Frontier then it is a different story.
Sent from my iPhone using Tapatalk