Nissan only made the Cummins powered Titan XD for a few years. It may become difficult to find parts.
I don't own a Titan, but Car and Driver didn't particularly like their Cummins powered Titan:
"We had a poor experience with a Cummins-equipped Titan XD during our long-term evaluation. Our long-term 2016 Titan XD left us stranded twice within 40,000 miles. When it was running, we did not appreciate the diesel clatter, thirst for diesel exhaust fluid, and the powertrain's general lack of refinement."