Q: 2006 pathfinder drives and shifts great but won't go into reverse... what is wrong?

Be sure that the gear shifter is actually selecting the reverse position; you can observe the manual lever on the transmission case to see if the lever is in the commanded detent position. If the transmission is in reverse, pressure tests, using either a scan tool or external pressure gauges, will reveal what is going on within the reverse hydraulic circuit when you select that gear. Among the possible faulty components are the reverse clutch and valve body components such as shift solenoids. Transmissions are not difficult to rebuild. There is no scenario whatsoever under which you must "replace" the transmission as what you have is absolutely rebuildable, were that required, either by yourself or others. If you desire a mobile diagnostic to pinpoint the issue, please request service via YourMechanic’s automatic transmission shift diagnostic service page. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.