So you must have an "LE" model with the AUTO mode transfer case. A problem that sometimes occurs is that there develops a poor contact between the harness connector terminal and the pin of the transfer relay, which controls the electric transfer pump. The AUTO mode transfer case has a wet clutch and uses both a mechanical pump and an electric fluid pump. When in AUTO, the electric pump operates at speeds below 35 MPH, at idle and also when in reverse; other than those conditions, the electric pump will not run.
The transfer relay, which, FYI, is the same P/N as the relay for the air suspension in the Armada, is located in the engine compartment, not far from the brake master cylinder, and is on a small bracket that attaches to the side of the engine compartment with a single, 10MM head (6Mx1.0) bolt. It will have two harness connectors plugged in to the single relay. In many cases, you can unplug the relay, clean up the pins on the relay with a small file, tighten up the terminal in the harness connector with a pin so it will apply more tension, and put it back together and erase the codes for the fix. In my case, it worked for about a year and the problem re-occurred due to corrosion on the relay pin, so I ended up having to replace the transfer relay, which was about $35 and is part # 25237-EA000.
If this is your problem, when you check for codes in the transfer case, you will get line pressure codes.