I see no one has replied. I was curious to see if someone knows.
I can’t say I know, but if I had to guess I would say front wheel sensors.
But if it is based on wheel sensors I would think he would have other symptoms.
1) There’s the obvious; error codes displayed.
2) On the 2006-08 Rav the rear wheels only drive if fronts slip, so I think he would be getting a dashboard display that VSC has kicked in and it’s gone into AWD.
3) His anti lock brakes might act funny during braking cause the systems thinks a wheel is sliding.
Just my thoughts.