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You would look at your scan tool datastream and observe VSS. You need to purchase the factory service manual for this vehicle in order to understand how to properly troubleshoot and diag. Is it your truck?
Good, then your customer can pay for it. Once you have that, you will have endless power to fix his truck. Unfortunately most of us auto guys don't have access to YMM specific data beyond 3500 series trucks. I already pay over $6k per year for my information & programming systems and let me tell you, I have to work hard to recoup my costs. Favors are fine as long as you aren't footing the bill. eBay is a great source for the manuals too. If you need help deciphering anything, I'm sure we can help, but I don't know of anyone personally on here that can access the heavy truck systems and provide limitless data. You need "description and operation", diagrams, TSB's, component and cell references as well as unit repair info if you are going to tackle serious repairs.