you know i don't know what to tell you, from living in seattle i can say that the 4runner handles just fine in the rain no matter what type/condition, so it's either your tires some suspension problem your not accurately telling us what the symptoms are or you just shouldn't be on the roads when they are wet.
have you come from a car or other low CG vehicle? how does it track on dry flat roads? have you checked your tie rods for play?
either start checking your suspension for flaws,get new tires, or slow down.....:boxer:
lol, sorry man can't help more then that without more information.
i can't figure out what would be different on yours opposed to everyone else besides the tires from the info you posted up.