It all deals with current load.............LEDs pull a fraction of the current that a regular bulb does. Since regular flashers are many times mechanical - they are made of two strips of different metals joined together into a single strip that bends when it heats up (due to current being pulled through it by the regular bulbs) causing the lights to go off - then they cool, the metal "straightens out" and they reconnect causing the lights to go on again until it heats up......) they HAVE to have enough current going through them to heat up. I have read that if you have two regular bulbs out of the four, the regular flasher should work. Since LEDs are brighter and light "quicker" than regular bulbs ( although it's just a fraction of a second, that can translate into the cage driver behind you seeing you 20' further back!), it would be safest to put THEM in the back!
..........Hope that helps!