jjg, I'm surprised you're still on stock suspension at that mileage

. I replaced the factory shock absorbers on mine at 49000km - they weren't doing the job then. IMHO the XL-7 rears weren't up to the task from the day they left factory.
Just remember stiffer springs can help level out the ride height, but they won't magically allow you to over load the vehicle past the manufacturer specs. New shocks (decent quality) and load leveling airbags (fitted inside the rear springs) could be a feasible option without ruining the ride when it's not loaded.
The local specs for ball weight are also only 85kg (187lb) or 140kg (308lb)
with load leveling equipment. That's with the heaviest duty tow bar available here, which is rated for a towing mass of 1850kg (4070lb). So, it would pay to check your owner's manual!
PS: There's a big spiel on here somewhere about gross combined vehicle mass - something it sounds like you could be exceeding?