I installed a simple 4 prong flat plug on my XJ so I can hook up my trailer, but when I plugged it in, the turn signals seems to lack the power to cause the trailer lights to do anything more than flicker. Is there a way to amp up the system so the signals legitimately flash? Any help is appreciated.

Replace your standard flasher with a heavy duty towing capacity flasher available at auto parts stores. It is loud when it blinks, but it will do the trick.