The driveshaft on my truck as well as every other newer F-150 that I have looked at appears to be aluminun. They are of a pretty good diameter, 5" maybe 6" not quite sure.
99 F-150 XLT Supercab, 4x2, Bright Red/Harvest Gold, 4.2L, Automatic, 3.55LS, 255/70R16 Goodyears, Captains Chairs, Keyless Entry, CD Changer, Tow Package
Accessories: SuperChip, Donnelly Compass Mirror, Lariat Wheelwell Moldings and Bumper Cap, Ford Splash Guards, Bug Shield, Bed Mat, WeatherTech Deflectors, Mobil1 Synthetic Oil
Modifications: Painted Tailgate Handle, A/C on/off switch