I had the relay for the OD quit in mine, it is under the dash on or near the left kick panel or so. I put a jumper in the relay socket from the OD switch on the shifter to the output to the trans. This reverses the operation of the switch but it works. I would not think there is a problem with your trans but this is the first time I looked at your issue.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.