I'd check out the EGR cooler. It's under the intake manifold and has some coolant lines running to it. From my research (I was looking to possibly delete it) it seems that these can leak an be a pain to spot.

70 Challenger
1960 Town Wagon P Pump 24v cummins project
06 Mega 3500 Drw 2wd
12 Durango R/T
18 Power Wagon
67 A100