I went through the manual self-diagnostic test for the transfer case and came up with no stored codes. Later, while idling and in AUTO mode, the 4WD warning lamp came on and the 2WD/4WD display on the instrument cluster disappeared. Also, I didn't have the ability to change gears in the transfer case. I drove home and in my driveway, I played with the transfer case mode switch and it went back to normal operation. Transfer case self-diagnostic mode came up with two codes: 11 and 12. My thinking at this point is that it is unlikely that both switches would fail exactly at the same time, so possible causes would lean toward a faulty control unit or low hydraulic pressure...possibly low fluid level (checked and saw no leaks), clogged filter (I would think that the filter is typically only serviced during an overhaul), or a bad pump...which would be an expensive fix.
I did some searching and found a 2-year old post at NissanHelp.com, also for a 2006 Pathy LE AWD which had some similar problems, including the same, two trouble codes: clutch pressure switch and line pressure switch. Posted stated he had problem for 2-years and found info on an Armada website where people were fixing the problem by replacing the transfer case relay. He noticed signs of overheating on the terminals, so he cleaned them up and tightened the harness terminal connections. This fixed the problem. This would make sense, because a relay with poor contacts or a bad relay could cause an intermittent condition. Also, this relay controls the electric pump in the AUTO mode transfer case, which operates at speeds below 35 mph to lubricate the wet clutch. Of note, both his issues and mine occur at low speed or idle and the failure was occurring in the "AUTO" mode setting. If the relay fails and the electric pump stops working, then a low pressure condition would affect both pressure switches and trigger the trouble codes.
So, my next step is to take a look at this relay and clean/tighten the terminals. Keeping my fingers crossed!