roy
we had that unstability feeling in the rear too under power and hard braking, (if you can get it on a hoist get a big bar in there and check all your bushes, our lower control arms were moving around and like duncan said the rods ends were also moving, so we changed them to polyurethane and it still actually had just as much movement !! and didnt fix the problem, we then put in solid alloy bushes in the control arms pivot points to check if we were on the right path and bingo much better, so we bought control arms with proper solid rod ends and the car is heaps better for it!
also with you going through brakes it means you are working them hard because you are driving hard and also proably getting great grip on semi's, we could get about 2 or 3 mm a side on the rear in bush movement (even though they looked fine) with a big bar in there, can you imagine whats happeing when you go hard under brakes and both rear wheels move back and outwards generally heaps of toe out and very unstable.
hope this makes sense ( getting late)
cheers russ