Low Oil Pressure when in A4WD
 

I have an 04' Chevy Silverado, 5.3L.
Last night I was driving in A4WD and noticed my oil pressure was at 0.
I shifted back to 2H, shut off the truck and started it back up, oil jumped to 20. Drove a couple of miles, went back to 30 (I seem to remember that normal oil was around 40 on the gauge). Shifted back to A4WD and it slowly dropped back to 0... Any ideas?

Checked oil level, it is at full.