What brand of lights were they? There are still a lot of LED lamps that do have resistors built into them, then there are the cheap ones (like from HF) that people around here cheap out on then get all sorts of issues because of the current draw.
Depends on which lamps you buy. Don't cheap out, and do your research first.
I can't remember. They were standard 4" with a decent amount of LEDs. I think I went with something that was DOT approved, just for visibility's sake. The reverse lights I put in were 2.5" LEDs I scored off of eBay. I was ready to have to install resistors if the blink speed was crazy, just happened to not be the case.
If I'm not mistaken, resistors are usually installed when the blinker current draw isn't as high as the relay expects, thus it blinks quickly, thinking there is a problem. The front blinkers are still regular bulbs, which have the correct power draw, so the relay is satiated.
Anyway, OP, you said resistors are all four corners? Did you install LEDs in the front, as well? If they worked fine before LED swap, there must be a bad ground somewhere. When all four are blinking, does the dash show both indicators flashing, or just the side you chose? I'm thinking you can probably narrow down the problem by disconnecting each side independently and testing it to see if it fixes it. Once you disconnect the corner that is a problem and everything functions as normal, as least you know where to check the wiring.