we have an 04 jimny which is standard but for no back seats. it has a witter detachable tow bar that was proffesionally fitted. when towing a single axle trailer all is fine regardless of the trailer being full or empty. however when pulling our light twin axle trailer (560 kg) get to 40mph straight nd level road and the whole rig is weaving and bucking most alarmingly. with load in the trailer it is worse. we have a friend with a swb vitara and but for that bieng a bit heavier and it's tow ball bieng half an inch higher if that it pulls our trailer without issue. we have also borrowed a trailer with a bit lower coupling and it is the same.
any help would be appreciated as this is baffeling me, cheers