Transmission not going in to OD yes is due to the motor not getting warm there is a sensor that will stop it for going in to OD tell the truck is warm.

MMM maybe the thermostat you put in it is crap.??? I have in the past bought cheap ones or..... maybe its not seated that well and its leaking around it. I there way I would pull it out and re-install another new one if I was you. You might even want to replace the radiator cap as well. Just for good measure.

1997 Montero LS with a motor swap a 1990 V6 12valve ported heads full 2.5in Ex I run stock tire size for Daily use, off road have 33x12.5 working on going trubo.