alecsey's link two posts above is a good one. Also, searching YouTube for Pathfinder / XTerra u-joint replacement is worthwhile.
Just in case anyone here is in the same situation as me and winds up needing to replace their universal joints, RockAuto.com has the Spicer u-joints for under $15. They also have the greaseable variety. They're listed under the Heavy Duty category of universal joints for your vehicle.
"Dana 51310X" (front shaft) https://www.rockauto.com/en/moreinfo.ph ... cc=1433013
"Dana 51330X" (rear shaft) https://www.rockauto.com/en/moreinfo.ph ... cc=1433013
"Dana 513101X" (front shaft) https://www.rockauto.com/en/moreinfo.ph ... cc=1433013
"Dana 513301X" (rear shaft) https://www.rockauto.com/en/moreinfo.ph ... cc=1433013
Although RockAuto has them filed under the Dana brand, the product description clearly says Spicer Life Series part.
Note the universal joints are cheaper through RockAuto than Northern Drivertrain.
Lastly, one thing to consider is using a 6-point (not a 12-point) closed ended wrench with an 'off-corner loading design' to break the stubborn nuts loose. 14mm for the front driveshaft, 17mm for the rear drive shaft. The 'off-corner loading design' might be just a sales gimmick (I haven't tried it yet), but it in theory doesn't touch the corners of the bolt, so it is less likely to round the bolt. An example wrench is the 14mm GEARWRENCH 81762 (google it or search Amazon).