does the engine seem to rev up fine when in neutral?
also, is reverse working?
ok, well there were some transmissions in your year model pathfinder that had an issue with the planetary set inside of the transmission. the planetary set would begin to lock up and cause exactly what your describing. it is hard for me to tell if this is what it is, but this is the first thing that came to mind.
I also know that once this planetary set locked up, it would cause all sorts of debris to go through out the transmission. this would make it almost impossible to rebuild. because of the price.
you best bet is to probably find a good used transmission from a junk yard with a 6 month to 12 month warranty. thats what i would do if this happened to me. make sure to have the transmission cooler in the radiator flushed if you end up replacing the transmission or else you will end up with this problem again.
I suggest you take get someone to drive it and verify it is a transmission issue. if it is, then your best bet is listed above.
good luck, and if you have any more questions about this issue, do not hesitate to ask here in this thread. i will respond as soon as i get it.