Could be a bad alternator, when they are bad they will drain the battery quick.... I'd check that out as well since you think it could be the starter.
If it isn't a bad alternator it is probably the starter, that would be a fairly accurate assumption with your symptoms.
2004 Ford F150 FX4
K&N Cold Air Intake System, Flowmaster Dual Exhaust System, 3in Lifted Bilstein Suspension, 35" Hankook Dynapro M/Ts, 17" Mickey Thompson Classic II Wheels and HID 8ks.