Toyota Repair: P0753 code Toyota, mechanical transmission, landcruisers

Question
Transmission works when operated manually (L1, L2, D). P0753 OBDII code was on, replaced both shift solenoids and wiring harness, changed trans fluid (very clean).  Could it be the NPS switch?  Or something else like the valve control balls getting stuck or ??????  It is a 1996 Toyota Landcruiser.

Answer
I just looked at the diagnostic procedure for this DTC and it looks like you have done everything to repair that could possibly cause this, except for a couple of other possibilities.
The possible problem areas in the manual do not mention a shift valve body problem. The other possible areas to look at would be the wiring between the transmission and the ECU or a problem with the ECU itself. Internal mechanical transmission failures are very rare on Landcruisers so I would start looking at some external problem, since it can be shifted manually it would rule out a mechanical problem.
The P/N switch could possibly be the cause, I have seen them get corrosion on the connectors and internally and cause electrical feedback problems in the circuit, good luck with this.