My 4runner did that. Turned out it was combination of lower control arm bushings and the outer tie-rods being worn. I'd hit a bump and the front end would almost... shimmy, or vibe hard. It was weird. It was noticeable and easily diagnosed on mine when the front was off the ground. I was able to grab the leading edge of the tire and rock it back and forth about an inch.
That movement coupled with grumpy old control arm bushings made for some odd sensations going down the road. Mine got so bad it killed one of my front shocks. It must've been bad in there for a LOOOOONG time before it even started giving me the signs. It handled well, didn't really wear tires or anything, until suddenly *weird noise/vibe*, like the bushing suddenly split.

Get 'er off the ground and check the tire play out, i'd say. Watch for slop in the tie-rods.