QX56 Suspension/Handling Problem

I have a 2007 QX56 and am looking for some solutions to the poor handling characteristics of the rear air shock and suspension. When driving on highway and I encounter grooved pavement, the rear end sways and will cause loss of control and excessive movement over bumps. On snow/ice pack, occasionally cause complete loss of contact.

On normal driving, hitting bumps in a turn, often causes the rear end to "jump". I notice that the suspension does not do a good job of "damping", instead, magnifies the motion. Especially side way motion, or "yaw".

Does anyone have suggestions for rear suspension modifications that would improve handling and reduce body roll and sway? I have changed the front shocks out with good coilovers and that has helped--the local 4x4 shop is recommending custom spring coils from Deaver to stiffen the rear end (I also tow 8000 lb rv.).

Finally, the air shocks will not hold the normal 750lb tongue weight below 30 degrees F....the dealer says nothing wrong, but in summer, I have no problems.

Thanks for any advice.