2003 Yukon with 200K won't start

[Raider Red]

We've replaced ignition relay, starter, battery, and fuel pressure regulator. My Yukon turns over for about 3 seconds then fires up. In the morning it's a little harder to start up, but once it does it runs smooth. Idle is consistently smooth too. I was told to replace the fuel filter so that's next on my list. Oh, fuel pump is working fine too. Could it be the crankshaft postion sensor? Any other recommendations would be greatly appreciated.