It could do with the MAF adapter also. Try to take it in for an inspection, not by Nissan (they're the stealership after all).
Do you have a check engine light at all?
I'm not sure if this will work...You can always try the throttle re-learn. Reason is that you said it's fine in neutral.
It sounds almost like what happened when I installed my intake manifold spacers. The accelerator pedal + rpms weren't matching up. So give it a try, the instructions are for 5th Gens 2000-2003 but it should work the same

Instructions courtesy of Nissan Works.
Accelerator Pedal Released Position Learning Procedure
1) Make sure the accelerator pedal is fully released.
2) Turn ignition switch ON and wait at least 2 seconds.
3) Turn ignition switch OFF and wait at least 10 seconds.
4) Turn ignition switch ON and wait at least 2 seconds.
5) Turn ignition switch OFF and wait at least 10 seconds.
Throttle Valve Closed Position Learning Procedure
1) Turn ignition switch ON.
2) Turn ignition switch OFF and wait at least 10 seconds. Make sure the throttle valve moves during the above 10 seconds by confirming the operating sound.
Note: If you experience a high or erroneous idle or any drivability issues, then perform the “Idle Air Volume Learning” procedure below. A stopwatch or clock with second hand will be needed to ensure you adhere to the proper timing specified in the directions below.
Idle Air Volume Learning Procedure
1) Start engine and warm it up to normal operating temperature.
2) Turn ignition switch OFF and wait at least 10 seconds.
3) Confirm the accelerator pedal is fully released, turn ignition switch ON and wait 3 seconds.
4) Repeat the following procedure quickly 5 times within 5 seconds.
1) Fully depress the accelerator pedal.
2) Fully release the accelerator pedal.
5) Wait 7 seconds, fully depress the accelerator pedal and keep it for approx. 20 seconds until the MIL goes off.
6) Fully release the accelerator pedal within 3 seconds after the MIL goes off.
7) Start engine and let it idle.
8) Wait 20 seconds.
9) Rev up the engine two or three times and make sure that idle speed and ignition timing are within the specifications.