It could be a CV joint on the drive axles, loss of a wheel weight,
steering component nearing failure, I would not drive it until you find
and repair what is causing it.

Like the other guys mentioned it could be anything. Let's start with the
simple stuff: when was the last time you checked the air pressure in your
tires? When the tires are balanced, they have a certain amount of air in
them. If one or more tires is severely low, the balance changes and can
cause the shaking you're talking about. Check the tire pressure first of
all, and while you're at it take a good look at the tires, tread, look for
bubbles in the sidewall, etc.

Then there's shocks. When was the last time you had the shocks replaced? A
blown strut can cause one wheel to bounce, and thus throw the car off
balance. Take it to someone you trust to have a look at the struts. Also,
have then check the strut mounts st the tops of the struts; these usually
make a CLUNK on bumps or corners when bad.

A worn tie-rod end can cause this also. When was the last time you had an
alignment? Also, a worn ball joint will cause this (although, in all the
years I've owned Toyotas, I have never had to replace a ball joint! Lucky?)
You can check this by jacking up the car (and placing it on jack stands,
with the jack as a back-up!) and wiggling the tires. If the tire moves
left/right and the steering wheel doesn't, then something in the steering
is most likely worn. Also, grab the top and the bottom and try to move it.
If you feel a kind of 'chinking', then the ball joint is worn and could
need replacing.

Good luck! This is the easy stuff. After that it gets more complicated!