Something is loose and/or out of balance. Have the tires balanced, check them for a "slipped belt", check the CV joints, brakes, rotors, struts, tie-rods and any other components that could have worn. I'd have to ride with you to know for sure, but you could have one or more of your tires that are out of balance. From your description, I doubt you have a torque converter shudder, since the lockup usually comes in around 40-45 MPH. Since it is at 50 MPH, I would think tires are your most likely culprit.
Replace freeze plug in a 1998 ford windstar ?
A 1998 Ford Windstar would use refrigerant 134a
The 3.0 liter 12v motor is very good. I have a 1998 Windstar with that motor and 235,000 miles and still runs like new.
On a 1998 Ford Windstar : The 3.8 liter V6 engine ( and the 3.0 ) has a timing CHAIN
On a 1998 Ford Windstar : ( # 16 / 20 amp )
Not likely - the 1999 Windstar was a rather significant updating of design compared to 1995-1998 models.
On a 1998 Ford Windstar ( 3.0 and 3.8 ) firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
On the underside of the engine.
It does not have one.
Mostly in the Windstar category - they all apply to the GL model
In a 1998 Ford Windstar : There were 2 engine sizes offered from the factory : A 3.0 liter V6 engine and the 3.8 liter V6 engine
Follow the LOWER radiator hose from the radiator . It connects to the water pump on the engine on a 1998 Ford Windstar