99 Ford Windstar - steering wheel shakes @ 40 mph

I replaced the rack and pinion, tierod ends, and tires last year....Any ideas? Thanks, Jim

First thing that I would do is to rotate the tires from front to rear......most likely cause is an unballanced or otherwise defective tire. If the steering wheel shake goes away with the tires on the rear (car will most likely still shake).....then it is a tire(s).