There is no real test that I am aware of that conclusively pinpoints it to the viscous coupler. For the most part it is the symptoms. The hotter it gets the worse it gets. Binding on turns especially if you drive in figure 8's at a parking lot. The tires will bind and chirp. Depending on miles a used one may not be much better. For sure these type transfer cases need 4 tires identical in make and size.