I doubt that putting air pressure into the main oil circuit would give accurate results.
I can imagine that there is a "normal" amount of leakage at various locations in the control valve under any conditions.
This normal leakage would obscure any leakage that might be escaping at a failed gasket surrounding the mail oil gallery where the trans case meets the control valve.
A more accurate test would be to put an oil pressure gauge into the main test port and see how much oil pressure is present at idle RPM and then at 2,000 rpm.
Ideal oil pressure would be near 150 PSI at 2,000 RPM. The absolute lowest pressure at idle ought to be something higher than about 90-95 PSI or so.
If you are seeing main oil pressure less than 90 PSI at idle when the truck is thoroughly warmed up, then you likely have a breach in the control valve mounting gasket.
One more thing you might want to do (in the form maintenance) is to check and replace the suction strainer.
It occurred to me that if the suction strainer is beginning to collect debris, it might be starving the pump on cold start ups and oil output would be reduced until the pump regains its internal pressure head. And of course, that would be worsened if the control valve gasket was failed.
Suction strainer is on the right hand side of trans case behind a 4 bolt cover plate.
Strainer part # AS35303-3601, cover O-ring # AS55931-3660.