1998 Ford Windstar

If it idles seemingly well, but doesn't have any power, try replacing the fuel filter. The engine may be starving for fuel.

Or, you may have an exhaust restriction. Remove the upstream oxygen sensor (sensor nearest the engine block) and drive the car for a bit; see if there is any noticible difference (despite the roar of noise).
Realize you'll likely get a check engine light for temporarily removing the O2 sensor, but it's all part of diagnostics. Have a code reader handy to clear codes.