Why do I have sway with a WD hitch and sway kit?

You need to weigh it so you have actual numbers. with a 19ft trailer properly setup the Grand Cherokee should be able to handle it. pump up the air pressures to max on the sidewalls, make sure the suspension is tight and spend some time tweaking the hitch. it may take a few trips until you find the towing comfort you are looking for. you can`t just hook up a trailer and expect it not to need any adjustments!