I looked at the video (which was done nicely by the way), before I read anything. I thought to myself... "WOW... those rear shocks are WAY too loose".

Fast forwarding down to your picture... it's hard to say if the shock is too loose or too tight, but because of the steep angle it's making it "act" too loose.

Think of it this way. If the shock is straight up and down, the axle separation feels whatever force the shock puts on it. As you rotate the angle of the shock (in any direction... rearward, forward, inboard, outboard...) it begins to loose effective force on the angle. If you were to think of the extreme where the shock was mounted completely horizontal, it obviously wouldn't have any effect on the up/down vertical motion of the axle housing at all.

Because of the angle it's mounted at, the shock can't control the up/down housing movement during any kind of launch situation. It's unlikely any shock could/would be valved to fix this at the angle it's currently at, which is why guys are saying you need to fix that first before you do anything.

The A-body spring segment length is fine. That truck has a fairly short wheelbase, so it will work without issue... meaning don't worry about the instant center right now. Fix the shock location first, and that will help you a whole bunch with the traction.

Right now since the shock can't control the body separation, it basically uses up all of the travel right away, and then once it's out of travel it just spins the tires.