Do you have 2WD or 4WD? having just the rear tires worn is a bit unusual for 4WD, but I guess it could happen if you didn't rotate them enough.
I would be hesitant to have tires with a mis-matched amount of wear if it's 4wd. There is a limited slip center differential, and it might be fighting the slightly different tire sizes, and if you lock the center it might stress it a bit and cause the tires to skip or slip.
In general the 4runner is as good as its tires in the snow. It has some electronic aids that help a little, but they can't give you more traction.
To get going, theres Atrac/Trac (4WD) or Auto LSD (2WD) that will apply brakes if one of the drive wheels slips and starts to spin to send power to the rest of the wheels with better traction. It might also limit engine power because it works best when the wheels are not slipping fast.
To keep you going in the right direction there's VSC (vehicle stability control). It measures the rate of rotation of the vehicle and the angle of the steering wheel, and it makes sure you are moving as expected. If you start spinning out, it will brake one or more wheels individually to try to correct your path. But it is still limited by traction, so if you are on ice it won't be able to do much.
There is an electronic lock for the center differential that's helpful to get going if traction is really bad, but for normal driving I prefer to keep it unlocked because the handling is more predictable that way. With it locked you tend to get a lot more understeer (going in a straight line when you are trying to turn).