Sway comes about as a consequence of a too-light hitch weight
. A trailer with little or no weight
on the tongue is asking for sway trouble, because the back end (that's about as heavy as, or heavier than, the front end) will want to come around on you. I found this out when I mis-loaded a little 4'x8' utility trailer one day long ago, and it gave me quite a scare.
So the question is, what about unusually heavy tongue weights? Well, a 400 or 500 lb. tongue on a vehicle rated to take that much tongue weight
should be no problem. That is way better than a 100 lb or 50 lb tongue, or even a negative weight
(like I did with the utility trailer!). Of course, if you were loading a Subaru up with that much on the hitch, you would be overloading the rear suspension and taking too much steering ability out of the front. But with an Envoy or a pickup or some other large vehicle, you should not need to worry. A sway bar or anti-sway hitch should not be necessary, either; but if it makes you feel better, by all means go ahead and use whatever gives you peace of mind and added margin of safety. It can't hurt!