Alright... well initially I thought starter, since it has started to whine a bit. All the accessories seem to work fine, it doesn't have the usual signs of a bad battery but I figured I check. They are red top Duralasts, took them to Autozone tonight and they checked 100%. Yea, I know, it's Autozone, but they've been good to me and the guys at this one seem to know their stuff. But I checked the batteries for free and a starter does seem to fit the symptoms.
That does sound familiar now that I think about it. I have been astounded at how long it takes to fire, even in warm weather when it was cranking fast. I guess I could have both problems? I mean I'm working with 156,000 miles and the assumption that nothing more than oil has ever been replaced on this thing.
You seem pretty sure about this, and I'm pretty clueless about diesels (obviously). Can you (or Byron) clarify something... looking on RockAuto I am seeing a "glow plug switch," that looks like a conventional round four-terminal solenoid. $50. I'm also seeing a "glow plug relay," that has two wiring harness connectors with some wide pins on them, and looks like it has a heatsink built into it. $150. Which one is the culprit?
So, what to try first? Or both? Both a GPR and starter seem to explain all the problems. It definitely is cranking slow (realllly slow, as in any time could be the last time it spins), as well as taking a lot of revolutions to fire, even in warm weather and when it was cranking hard. If changing both fixes my problems for years to come, then no big deal, I'll do it. Or I'll sell it and buy a Dirtymax.

But I am leaning towards keeping this truck for a while and I need it to reliable, so I'll do whatever it takes.
Edit: and nice post-whoring, Brian.