Oil pressure runs much higher than coolant pressure...........oil cooler is what I'll put my money on!

Well, turns out I don't need to worry too much. Kyle, the VM Motori trained tech at Florida Detroit Diesel Allison in Tampa tested it during the timing belt job (they are excellent, by the way), and it turns out it isn't oil. No oil in coolant, and no coolant in oil. Head gasket is fine, and he did an excellent job on the complete timing belt job, including a new thermostat, graf water pump, hayden fan clutch, improved serp tensioner, etc. He thinks maybe some grungy stuff let loose from a hose or something and collected on the upper walls of the coolant resevoir. There's nothing actually mixed into the coolant. The coolant tests fine, and the truck runs like a champ. So, I'll keep my fingers crossed, and start racking up some more highway miles. The thermostat gasket was leaking for a bit, and was most likely the last source of my coolant loss (the other was lower radiator hose), so I'm happy about that. My thermostat was definitely starting to stick open, and now works flawlessly. All is well in CRD land.....for now.