I'd definitely would have the ball joints replaced if the boots are torn, ball joints can fail suddenly and the spindle can pop loose from the control arm. With the vehicle on the ground have someone move the steering back and forth a couple of inches will looking at and feeling the tie rod (inner and outer) and ball joints. Usually permanentky lubed joints will make a noise (squeek or groan ) when they are getting bad. Also does the steering wheel from right to left or front to back. Front to back (a general vibration) usually but not always means a balance issue needs to be looked at. Right to left could be alot more things from: low tire pressure, overtightened lug nuts, debris between rotor and hub(usually after a brake service), loose suspension and steering components, separated belt in tire, bad motor and/or transmounts, aftermarket wheels that aren't hubcentric and more things I can't remember at the present time. The shake improves when it wet is probably because the tires can slip more then on dry pavement. Worn components or a bad alignment can cost more money by not getting it fixed; i.e. multiple worn tire replacements, poor fuel economy and lose of steering resulting in an accident.
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