Add to your list of possibles is ECT (engine coolant temp) sensor. It won't allow trans to shift to OD until engine warms up. Your heat gauge will look normal but the sensor may be off spec. Have you been Having any drivability symptoms other than trans?

You can test the ECT on the engine. Test resistance cold then with hot engine and see if within spec. My 92 specs are 2.37 +- .24 K ohms at 68F and 290 +- 32 K ohms at 176F. Yours will probably be the same as these are usually standard thermistors.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles