Welcome to the forum.
Sounds like a sensor problem in the transfer case, or a servo failure. There are a few things that you can try.
First thing is to count the flashes. The TCM will light for 2.5 seconds then pause off for 1 second, then flash codes 0.5 seconds apart, pause for 1 second, then flash the second code 0.5 seconds apart. Count each code - report it here, there are 15 possibilities.
Other options:
1) Remove the electrical connections from the plugs on the transfer case and reinstall them after giving them a spritz with WD40 or similar. Inspect these wires - there have been occasions when they've been damaged with the same results you're experiencing.
2) Lie under the vehicle (supported by jack stands) and have someone turn on the ignition (do NOT start the engine) and engage 4WD while you listen to the transfer case, in particular the servo. If you don't hear anything at all, it could be a transfer relay under the bonnet (check the IPDM/ER black box just behind the battery in the same fashion as you check the servo). If the relays are working, the servo might have failed. It's happened a few times before.
If the servo has failed, test the normal operation like this: remove the servo, turn on the ignition (do NOT start the engine), have someone press and hold the brake while you insert a large flat-bladed screwdriver in the slot and turn it 90 degrees. This will engage 4WD. Examine the display - the four wheels should be 'lit'. With the brake still on, turn the servo a further 180 degrees (for a total of 270 degrees from where you started) to engage 4LO. Examine the display.
