As others mentioned, the TPS is a common problem that can affect shifting. A bad TPS can cause it to stay in first, or upshift pretty quickly. Failure to upshift out of first can also be the output shaft speed sensor. The sensor is on the drivers side, near the rear, tucked under the mounting plate for the xfer case shift linkage. Follow the wiring on the drivers side back. It's a pain in the butt to get to.
If you have a multimeter, the sensor is pretty easy to check. The sensor is basically a magnetically activated switch and gets closed once per revolution when a magnet swings by it (edit - this is for Renix only, later uses a more reliable inductive pickup). Hook up the meter, set on resistance to the sensor. You can either measure at the TCU connector, which would check more of the wiring, or check at the connector for the sensor down at the tranny. Then you just need to turn the output shaft and see that something happens. If the sensor is connected and powered, you should see it jump between 12 volts and zero.
You could stick the tranny in neutral and lift a tire and turn it. That would be the easiest to go slow and see the switch opening and closing. Or if your meter is fast enough, put the xfer case in neutral and the tranny in neutral or drive and the output shaft will freewheel on its own with the engine running. WIth the meter on voltage, you might see it averaged out as a lower voltage.
