Abe, there is one way to test it in the driveway. Put it into 4WD and jack up one front wheel and try to spin it. If it doesn't spin, it works. But bear in mind that you will probably have to have the engine running if it is the NP231 transfer case since there is a vacuum engagement of the axle lock on most of these models. So you can see if the factory jack, which should be there anyway works too.