Having done several rural routes over several years as a subcontractor, it is of PRIME IMPORTANCE to maximize fuel economy- that is, if the point of driving the route is to make money moreso than waste your time. (Gotta just shake my head at the local pizza delivery genius' that deliver in their mud trucks. There is no hope for humanity is there.
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I've done these routes through MN winters, on rural roads (paved and gravel and dirt) and I can say without question that 4WD or AWD is NOT NEEDED (might need it if you have mountains tho'). For the low traction times you will want good tread and the heavy vehicle weight bias on the driving wheels though i.e. fwd= weight forward, throw as much of the load on the front seat and floor as you can; rwd= weight back, keep as much of the load in the back seat as you can.
The route can be completed much, much faster when your arm hanging out the window is at the same height as the boxes. Having to reach down from a tall vehicle really, really slows things down- you might even have to make complete stops.
Narrow vehicles rule!!!! When I used a Super Beetle I could deliver out of both sides of the car down deadheads, then cruise without stopping on the way out. Yeah baby!
I wouldn't even use a cosmetically "nice" vehicle for route work- it's tough on 'em. You want the lightest weight possible, with only the carrying capacity needed to get through the route. It's easier on brakes and fuel econo and maneuverability that way.
P.S. Four cylinders or even less is what you want. SERIOUSLY.