Left Turn Signal Problem - Both rear lights flash

Yep just troubleshoot like the others above say. If you have to pull all the bulbs. I4 sounds like one of your lights/pigtails the positive charge is shorting to your ground side.

 

So if the pigtail is plugged into the truck but not a trailer, it does it, but if you unplug the pigtail from the truck as well no problem? But a new pigtail and problem solved

 

Oh yeah, if you haven't done so in the last few years, change out the red and blue airlines as well. It really sucks when one cracks on you

 

If, when you do that and the bulb is still in the socket, it is most likely a shorted bulb. If the problem is present without a bulb in place, then it would be the socket.