Since you said it spins with greater resisitance with a wrench in addition to with the normal cranking, I wonder if it's a manual tranny and the extra friction could cause it to hang up a bit. Dioes it spin hard when in neutral as well? Also, maybe try jacking the rear wheels up off the ground and spinning it as well. Is the fuel mileage normal?
Maybe it just has such good compression... wouldn't that be nice!
Adam