Sorry, I just noticed this topic -
KS is correct - a different flasher is exactly what you need.
The OEM flasher works on the basis of current draw. Since the typical LED arrays used in aftermarket lights draw less current, the flasher speeds up (this is backwards from most cars, by the way). The resistors that someone mentioned to you are used to draw more current, thereby making the blinker slow down.
The better solution is to install the "trailer" flasher, as the flash rate on this unit is independent of current draw. Take your existing flasher into, say, a NAPA so they can match the pin configuration. The number mentioned in the post that "KS" referenced is #552, but you should still take the existing one with you to be sure.
Best of luck,