any idea how one goes about checking them?

I should also note that the dash lights for 4wd do NOT come on when I turn the knob. from what I've read, these are triggered by the output shaft on the transfer case moving to a given position, so even if the hubs were not engaging b/c of a broken vacuum line, the transfer case would still be shifting which would trigger a dash light. since this is NOT happening, I'm thinking the transfer case motor is never turning the shaft to begin with.

So if that's the case, the problem is somewhere between the dash knob and the transfer case shift motor.