Rookie - 6.7 bad battery or relay/starter?
 

Got my first diesel 4 days ago used. 62k miles 2014. Took it to TWO different mechanics not related to dealer and had them check it out both agreed it was in basically flawless condition. Carfax was also perfect. Then this happens........ It ONLY occurs on the first startup after being dormant for a few hours (usually longer than 3) and the severity of it depends on how long its dormant. Although in truth the video i took above was probably the worst. Say i crank it and go drive for 20 minutes then get out for an hour and come back itll crank immediately. I live in lower alabama where its 49 degrees outside so its not the cold.
Anyone got any ideas on what it is? Id like to know what I'm about to get myself into before i take it to the shop

gee i dont know were to start...i would try all the grounds for clean and tight...also check your cables for ohms ...should be no more than .02 ohms per foot...scott

I'm resonably sure it's not the batteries! It has to be either wiring connections (I would start with all the battery cables!), a bad relay, or a computer.
I would start checking all wiring and connections! Battery cables, then move to wiring to the starter.
I like testing for a good connection under a load. Easiest way to explain this is simply put your digital volt meter positive lead on the negative of your battery's post (not the clamp or wire!). Then put the negative lead on the engine block next to where that negative cable bolts to the engine block. Have someone crank the engine. To do this test, you need to have a normal (ie. Long) crank of the starter. What your measuring is DC volts. You want less than .1 volts / per connection. (So: .1 volts max at battery connection. And .1 volts max at the engine connection would add up to a max of .2 volts while it is cranking.) (In practice, if you see more than that, you have a bad, or dirty connection or a bad cable.)
This test applies to all wiring. But must be done under its load to measure any voltage drop. When you see more than .2 DC volts measured under load; you know you have a problem you need to address.
On smaller circuits, don't ever poke a hole in wiring insulation to get a connection!!

This truck has factory remote start. Does the same thing occur when you use remote start?
It also has auto crank (bump the start position with the key, let go [release key back to ON position], and computer will crank until engine starts running!). What happens when you do this?
I don't know what all is in this ignition switch, but that would be on my list of suspects also.