I'd check under hood first. Two 12mm nuts to loosen the cable cap/guide and check that end of the cable. If strands are frayed/broken but still mostly intact, adjustment at the frame might buy a little time. Make sure the rear shoes are adjusted properly before you mess with the e-brake cable adjustment under the cab.
I know what the problem is with my e-brake.... just haven't repaired it yet. I'm carrying around a piece of broken cinder block to kick under a tire!
