if you put the tester on the 2wd side of the actuator, then put it into 4wd, I would imagine that you would get a little vacume, simply because you have sealed that side of it, and the engine vacume is trying to move the diafram to the 4wd side(basically, making the 2wd side bigger, and because it's sealed with the tester, it's going to show some vacume.)
here's how I'd do it,
with the truck in 2wd, and the engine running, remove the vac line that engages the CAD unit, and put the tester on that (4wd) side of the actuator. there should be no vacume.
then, reconnect the vac lines and put the truck in 4wd(make sure the shift fork has moved before continuing). remove the vac line from the 2wd side of the actuator, and attatch the tester. again, there should be no vacume.
sorry if this doesn't make much sense, if someone knows what I was trying to say, and can explain it better, please do.