What is causing this?

Sounds like PH and kh covered the ones I'd consider the most likely. A bad alignment would cause the car to pull one way or the other, but wouldn't cause shaking at a specific speed. If it's an imbalance, and the tires haven't worn badly, then it's a simple/inexpensive fix. Tie rods and such are going to run you some bucks, but you shouldn't put off having the work done. It's mainly a safety issue, but also the longer you put it off the more damage to your vehicle, and the more pain to your pocket.

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Probably tire imbalance. A quick check would be to see if you lost one of those clip-on weights, but it's easy and cheap to get them balanced.

sounds like a bad tie rod ball joint check them out and get it realigned i would'nt rule out a bad king pin or king pin balljoint in which is usually on front wheel drive

get the tire balenced

exactly. also could possibly be blown struts.

could be either or both.