Have you looked at your motor mounts to see if the rubber inserts are shot...as yours should be done by now. There are 3, one top side passenger side, two lowers and there's yet another on the manual tranny. IF those are gone, you will get that clunk as the engine rocks as it gains/loses load.
Ok, what I'd do is get an assistant and jack up the driver side front, on jack stands (this will prevent car movement if your assistant screws up on the brake). Have assistant start car and press firmly on the brake pedal (all the time). With your gloved hand resting firmly on a valve cover, have them shift in and out of reverse. Enough to isolated any engine movement from the noise/clunk. If it is not the engine moving then...
Initially, use a short block of wood and hold it firmly against the driver side cv axles shaft. with one hand very close to where the wood touches the shaft (you can do this from the ground, in front of the wheel/driveshaft (not behind!)). Have assistant shift into reverse and back to park, several times. If you are confident the axle won't spin and you are unsure of the clunk and want more accuracy, use the palm of a gloved hand (do not wrap fingers around the shaft!) and press hard against the shaft. If there is any movement in either the inner or outer CV joint, you should feel a very mechanical clunk. You may also wish to repeat for passenger side for comparison.
Thanx for the info. Made the test and no play within the shaft. Seems like something within the tranny where the shaft enters. Hate to do it but think I'll get and appt. from Nissan and let them try to tell me that the tranny is bad and needs replacing.
Ok, that's an unfortunate result, but at least the work you did was fine and the dealer diagnostic fee is now justified Let us know how things progress...there are lot of people out there tooling around with the same transmission.