I actually have a brochure for the 1989 trucks, and it says that the 2WD F-350 is supposed to have a monobeam front axle, and the 4WD F-350 has a solid front axle (4WD F-250 still has TTB).
So it goes:
Bronco: 2WD-Twin I Beam 4WD-TTB, Front Coil
F-150: 2WD-Twin I Beam 4WD-TTB, Front Coil
F-250: 2WD-Twin I Beam 4WD-TTB, Front Leaf Spring
F-350: 2WD-Monobeam 4WD-Solid Axle, Front Leaf Spring