The Nissan Path

I have a 2008 Pathfinder S, have a fault code of P0734. Transmission seems to stay in one gear now. I drained the fluid and changed the filter but no luck. I assume 4th gear is gone.
I'm looking at replacing with a used trans or rebuild it myself. Any help please

Causes of this DTC may include:
Low or dirty transmission fluid
Mechanical failure inside transmission
Internal blockage inside transmission main control
Faulty Shift solenoid
Faulty transmission control module

Considering the fair number of solenoid we see fail on these transmissions, have you ruled out a bad shift solenoid?...or, a dirty valve body or faulty TCM? It would be a lot less expensive than a replacement trans or overhaul, as well as a much easier repair, if it were one of those as opposed to R&R the entire transmission.

I changed the fluid, filter and ruled out the solenoid......