Not sure about your 2012 6.2 but on my 06 6.8 I found the vacuum line going from my PVH Valve to the hubs had melted by the exhaust manifold. The vacuum tubing melted together so I had vacuum when I tested the lines at the hub and I of course had vacuum at my PVH valve I didn't notice this until I had to replace the starter. I ended up bending up a piece of 1/8" stainless tubing from my work to make sure that would never happen again. I also disassembled, cleaned, inspected & greased my hubs while I was at it and everything worked great after. This might not be any help on your 2012 but I thought I would at least pass on what I discovered.