If you're looking for the 6.4L used, it may require a long search and a longer road trip. If this one will be dual purpose (work and personal use), have you checked out a Ram Tradesman package with the 6.4L?
Dealers near me most always package them with the most necessary upgrades, and a 2500 Crew 4x4 comes in around $ 35K after rebates and discounts. (I think you mentioned 3500, but seems most 3500s are ordered with the diesel).
Example:
2017 Ram 2500 Crew 4WD 6.4L TradesmanWhat's not to like?
For used Rams, this VIN lookup might help:
Ramforum VIN tool