I'm also fresh out of C4.

Like Torqmada sez it looks like you're not getting enough juice to the starter.

When I had the same problem on my 87 2.9 I found a bad battery cable with this VERY simple little test.

I used one side of a set of jumper cbles & jumped from the positve battery post to the starter relay. No joy, it still would'nt crank.

Jumped from the negative battery post to the engine block & when it fired right up I knew the negatve battery cable was shot.

8 bucks for a 6 inch battery pig tail at Kragen & problem solved