Tachometer has stopped working Safari Sahara Diesel

So I went to a DSU Peterbilt service center on the way home today. They couldn't find the part. The sensor is actually mounted at the rear of the transmission, just forward of the bell housing, so the guy told me he thought I should go to an allison service center, and also that it was probably an engine speed sensor, not a crankshaft position sensor. I don't really know the difference, but I'll be going by the allison shop on thursday.