My truck won't go into 4 wheel drive. What's the easiest way to test the vacuum shift motor? What else should I look for, the transfer case is engaging?

 

i would bet money it isnt the transfer. just take the actuator out. its only four bolts. then put the transfer in 4x4 and see if you can spin the drive shaft. if that works then its your CAD or an axle shaft broken. with the truck running take it in and out of 4x4 with the actuator out. it should move.
the thing is probably just grimed up and wont move the collar on the axle enough.

 

It is easiest to make sure that you have vacuum at the actuator first. If you do, then pull the CAD and take a look. Be careful undoing the bolts, they have a tendency to break off although you can usually get vice grips on them once the CAD is off.

If you want to check the tranfer case, put the truck up on blocks and engage the 4wd lever and see if both driveshafts turn together.