I never towed any distance with my TJ or my JK, so my personal experience is limited. Checking for extended bump stops are a good place to start. Keep in mind, while many tow with a lifted truck, not many people tow at the weight limit with a "lifted" coil sprung vehicle. Here are several things to look at: (You better do this your self, dealer techs usually just look for obvious problems.)
Has the Jeep been lifted with an entire kit, or just longer springs. Meaning are the control arms stock, or aftermarket? If they are adjustable, are they both the exact same length? Different lengths will put the shocks in a bind, as will bent control arms.
Are all the bushings on the rear control arms in good condition?
Are the panhard rod bushings in good shape?
Has the panhard rod been lowered or replaced with a lowered unit to compensate for the altered axle position?
A lift will allow increased lateral movement even without good bushings. If you replace the bushing, use urethane rather than rubber to help keep the movement in check.
This may sound crazy, but check all the bolts for tightness and "play" in the brackets. Jeep used a smaller bolt in the JK's suspension than the hole in the frame and axle brackets, apparently to facilitate easier assembly. (Something like an 11 mm bolt in a 1/2" hole, don't ask how I know.) These bolts should take some force to install, if you replace them, use grade 8 bolts.
The ProComp ES9000s are good shocks but you are asking them to deal with both increased "un-sprung" weight and "sprung" weight, I'm not sure they were designed with that in mind. Make sure both the top and bottom of the shock are able to move slightly on their studs and bushings.
Any shock binding will load the shaft to one side in the shock tube.
If all above is correct, Just as Doug says maybe a WDH would absorb some of the increased "shock" associated with the sudden loading and unloading of the rear suspension while pulling a trailer.