This is a common (but little known) condition that affects many Windstar and Freestar (which uses the Windstar) steering shaft.
If you haven't let the stiff condition go on too long, you probably only need to lubricate.
Lubricate it by spraying alot of WD-40 on the steering shaft's lower U-joint immediately above the steering rack spool housing.
Spray it good, turn the wheel 180* to either direction and spray it again.
Then work the wheel back and forth form full-stop to full-stop.
After that, check to see how much play you have in the joint ... you do this by doing a "dry park" test.
Dry Park Test:
- With vehicle off, in Neutral and the full weight of the vehicle on all four tires (i.e., front wheels loaded), have a helper turn turn the steering wheel slightly while you feel the U-joint for play.
Unless you are a really skinny anorexia guy, you'll need to have the front wheels on a ramp.
If you see a lot of play in the lower u-joint or the WD-40 lubrication described above fixes the problem for only a short period of time, then you need to replace the steering shaft.
Steering Shaft R&R: (Removal & Replacement)
The steering shaft is a Ford-only part... so you need to buy it through Ford Dealer... as of 2013 they are going for $150-$200.
But the removal and replacement is easy. The unit is attached with only a pinch bolt on top (inside the cab under the dash by the shaft's upper U-joint) and a pinch bolt under the vehicle where you just sprayed the U-joint.
Pinch bolts are easy... lefty loosey, righty-tighty.