One easy way to check is to compresse the rear suspension, i.e. push down on the back of the car. Push it down repititously three times, then release, if it dosn't bounce back up and hold, the rear sruts are prolly bad and causing some sort of sag. This will not cause the gap to go away in the front, but it will give you a better ride and probably will caus ehte sag to goaway. Nissan's are also well know for having a bigger gap in teh front then the rear, even the newer cars are like this. with my KYB GR-2's and Intrax springs, my gap is the same front and rear, both tires are IN the wheel well, I have 205 40 16's.
John