Why is there so much confusion?

I didn't have any problem when I installed them on my 2007 E-450.

I installed the Bargman LEDs on my '03 E-450 and had to change the flasher for them to work right.

Careful about the front light changing to LED. If the chassis has lamp outage, you may need to install resistors to keep the system from saying you have lamp out.
This would happen with clearance (parking) lamps as well as turn signals.
I had to go back to conventional in front as the ballast resistors used power and got very not warm but hot!

As already mentioned, the "hyperflash" is caused when the draw of the LED lights are so low it causes your vehicle to think the bulb is out. However, this is a case of may or may not since you're only changing the rears. The fronts and sides may (emphasis on may) still pull enough to avoid the problem. But, if planning to replace them all with LEDs, might as well also change the flasher to an electronic version now so you don't have to deal with it as you change out the bulbs to LEDs.

I would check to see what flasher you have. Some LED lights will not have sufficient resistance to make the flasher work.
I would and did change my flasher to an electronic version that does not require much load to make it work. Then order the LED lights that you want.