[kipps]

GM never equipped a truck with three tanks from the factory. The pickups got one or two of the side-saddle tanks, and the Blazers/Suburbans got the rear tanks. No pickup ever came with a rear tank.

Somebody obviously added the rear tank from a Blazer or Suburban at some point in the past. How did they plumb the fuel filler? Through the bed? Behind a tail-light?

If you intend to keep the rear tank, I'd suggest you pull measurements, and try to figure out which tank was used. There were two different sizes; 30-gallon and 40-gallon, IIRC. There shouldn't be anything fundamentally different about the sender between gas and diesel, as long as it has the correct size and number of ports. A diesel sender will need to accommodate a fuel return. Most gas sender units didn't bother with a return.