I finally got around to tearing into my transmission over the weekend. I was hopeful that I would just simply replace the (Front Brake) solenoid and it'd be all good, but I ran into a bit more resistance once I got deeper in the transmission. Sorry for the lengthy post, bit maybe this'll help someone in the future who is experiencing this issue.
I purchased this, our 2nd Pathfinder, the week of Thanksgiving for next to nothing knowing it had a transmission code stored, but it seemed to shift strong so I was hopeful that it was just an electrical or programming issue. Worst case, for what I paid, I figured it'd be worth putting a reman'd transmission in it when the time came. I did the bypass right away and drained/filled fluids and didn't see any indication of cross-contamination.
Shortly after this, I lost the use of 5th gear as explained in the post above with the P1757 code coming on intermittently but being able to clear with my ScanGauge. I got around this by pushing the O/D off button and simply driving it for a while with just 4 gears. I don't drive on the highway much so it didn't really bother me.
Fast forward to last week and the transmission went into limp mode - P1757 and unable to clear the fault with my ScanGauge. From what I could tell, it got locked in 4th gear. I drove it home and tried to unhook the battery and clear the codes but when I tried to drive it again, it seemed to be locked in 2nd gear and there seemed to be a lot of vibration internal to the transmission.
I ordered the new solenoid for the valve body and picked up some dielectric silicone and some electrical cleaner. My hope was that it'd be the solenoid or maybe an easy fix on the circuit board - similar to what I've seen on some of the Z cars when researching this issue with this family of transmissions online.
I drained the transmission and dropped the pan.
I then pulled the valve body out and removed the solenoids and the transmission control module. I tested the solenoids and they all seemed to be within a close range of each other on resistance.
I then turned my focus to the Transmission Control Module (TCM) and split it apart hoping to find a loose or broken connection. I didn't notice anything like I'd seen in my research, but once I popped the covers that connect the CPU (or whatever it's called) to the rest of the circuit board, I noticed a large amount of 'gunk' built up there.
My guess is that there was either coolant contamination from a previous owner or somehow it just built up corrosion over time. Tough to say since I bought the Pathfinder with 151k miles on it. Long story short, I spent about a half hour tediously cleaning out the 'gunk' and inspecting the connections to the CPU to find that one of the connections wasn't terminated. Seems as though it corroded through the connection between the pin coming from the CPU and the tab on the electrical board.
All of this took much longer than I anticipated so I apologize, but don't have pictures of after it was fixed - it was getting after midnight and I just wanted to get it buttoned up for a test drive. I got the connection cleaned up and re-soldered, covered the connections with the dielectric silicone to hopefully prevent future/further corrosion and put everything back together. I put in the new front brake solenoid since I already had it on-hand and filled the transmission with Valvoline MAXLife ATF - it took about 8 quarts of the 12 I had on-hand but I'll continue to monitor the levels for a few days.
To my surprise, I took the Pathfinder for a test drive and it works great. No codes, the transmission shifts well, the 5th gear works and I'm experiencing none of the vibrations I was getting previously. I'd say it shifts as good as our 2010 does. I am by no means an electrician and have minimal experience soldering from the few times I did it in college like 10 years ago (mechanical engineer). I'd say that if someone is competent with a wrench you can do this repair yourself as well. I've got around $100 into the new solenoid and transmission screen and around $70 in transmission fluid. I figured it'd be worth a shot vs. sourcing a reman or used transmission, plus doing the removal/install.
Hopefully this'll help someone in the future!