With the extra propane(?) tank you're carrying in the rear, I don't doubt you hear some creaking and groaning going on. If I recall, you've added something like 300# to the trunk. What the actually translates to for stresses in your vehicle, I'm not sure, but I bet you could use a few extra gussets here and there. Rear torque boxes at least and maybe a some extra brackets around the shock bracket.
Gotta say, I've never seen a car tear up the shackle area, so not sure how much of an improvement those would yield.
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Herman, a while back I found these pictures of reinforcing the rear:
