you was told right ,you can start the engine and check the vacuum on the line going to the actuator for it,one line will have vacuum on it in 2 wheel drive and the other will have vacuum in 4 wheel drive,its best to test this though with the engine running,and you also have a vacuum switch on the transfer case that if it goes out it wont send vacuum to the actuator,9 times out of ten though its the actuator that goes bad, make sure the lines aren't bad also,if they loose vacuum it wont kick it in 4 wheel drive either,and yes it is an expensive part,i have replaced a few of them,good luck on it.