It's hard to see from this one picture and the angle -- but it appears you may possibly be bottoming out the shocks. You said you had 3 pretty good sized boys in the car which would also be a contributor to that.
Tie a piece of string around the shock "rod" - and push it down against the top of the body.... then go out and hammer it just a bit so you're loading the rear end... come back and see where that string ends up as it's pushed towards the top of the rod.... Do this on both sides because that will be measurable to see the loading as well -- left vs right side.
The shock should have sufficient length so that you have an equal amount of compression and extension... and when sitting with weight on the axle (as though it were on the ground on its tires) the exposed rod should be about half of the total (so 2" of compression and 2" of extension = for 4" of total travel)...
Put the jack stands under the axle --- and then measure the exposed rod length. It looks short to me.
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