Dodge most certainly did make a dedicated cab/chassis. You can verify this by the cab to rear axle distance as well as the flat profile of the rear frame rails. As Gravel Rat says, if you don't see these features it is a converted pick-up and not worth the headache.
A 98 is not a 24 Valve unless it is a 98.5. At 110k it should have all its OEM issues sorted, but take it to a Cummins shop first and have a fuel pressure test performed. I have 3 of these trucks and have done very well by them all.