After much frustration, we're back on the road. The viscous fan was, indeed, the culprit. I ordered what Amazon said was a compatible fan clutch, but learned after it arrived that '03s and '04s have the clutch and fan together as an integral piece. So that went back to the warehouse. Then I ordered a fan from our favorite parts store (I don't know the etiquette around here about naming retailers), but they packed it poorly, and it arrived with two broken fins. An angry phone call led to them agreeing to overnight a replacement, but they didn't do so until I called again a week later demanding to know where the replacement was. So now the fan is in the truck and I'm running nice and cool. I just need to return the water pump I bought. It has a steel impeller, which another thread here taught me is undesirable, so I didn't install it.