I would replace the flasher can just for a check. Be sure to use a 6 volt can of the right type. Six volt LED cans are available as I have them. Unsure of your problem as the left front and rear, and right front and rear lights are in parallel. Nothing I can think of short of a RC circuit (resistor/capacitor) which would delay the 'bleed off' of the 6 volts through the bulb would cause them to blink at a different rate. Maybe one is brighter than the other due to high resistance to ground but not blink rate. One set or both could not be LEDs could they? If you have LEDs somewhere, get back to us as that could change the game.
