I have had a caliper lock up before but not on my rav and it didn't really cause any shaking...this would be very easy to prove tho if it does it again...once it starts shaking going down the road immediately pull over (pay attention to effort needed to get it to stop)...once pulled over use the back side of your hand to touch all 4 wheels...if a caliper is getting stuck one wheel will be significantly hotter than the rest...could also be a brake hose collapsing and not allowing brake fluid out of the caliper...