Actually, the only time an axle will slide sideways is if both wheels are slipping. A non-slipping wheel provides lateral stability.
OP, the 4runner is geared low and the V8 has a lot of low-end torque. Translation: when you goosed it hard you broke both rear wheels loose and lost control of the rear end. The center diff provides roughly a 55% rear 45% front torque split, and the rear end is lighter, so breaking the rear loose on slick conditions is entirely possible to do. VSC and traction control should have dialed back the power pretty quickly, but maybe you got ahead of it.
A little lighter touch and you'll be fine. I love the way mine drives on snow/ice. (If only we had some here...)
I really agree with your assessment that a light touch is very much required.
There's a bit of controversy about the real torque ratio F/R, as that I've seen 60/40 R/F, respectively, more commonly noted.
Based on the way the truck handles, I would certainly think 60/40 is very reasonable as that it drives like a rear wheel drive truck that can torque steer the front axle.
If the sway bar and rear endlink bushings are in rough shape, the subtle weight movement in low traction conditions can cause the truck to slide out unexpectedly. I think this happens because the wheel that should have the weight (the outside) wheel, loses all its traction due to road conditions, and there's no weight on the inside wheel to compensate... thus, the truck slides. A good sway bar setup prevents weight from shifting around so much (and thereby unloading the inside wheel), and the truck will stay flatter and grip with all 4 tires, rather than the outside two.
When I replaced my rear endlink bushings, the truck felt much more planted on dry pavement. I don't think it'd be a stretch of the imagination that ice/snow condition traction would be dramatically effected by these subtleties.
When the rear endlink bushings are worn out, the sway bar is ineffective because the subtle movements of normal driving do not engage the sway bar at all... it merely moves the endlinks within their "slop" caused by the worn out bushings. So weight can move around from wheel to wheel without constraint by the sway bar system. This is less prevalent on the front... but as the topic of the thread is the rear stepping out; I would think this should be a consideration in the discussion.
The electronic nanny systems will only make up for driver error and physical/mechanical problems (and/or some combination of the two) to such a point... beyond that point, bizarre and dangerous handling problems will result.
