I just put in some LED's on my 00 GMC and when I put the blinkers on the blink...

LEDs draw much less power than regular bulbs do, and the flasher module that does the blinking is "fooled" by the low draw. (It might affect your turn signals, too.) There may be a special flasher module for LED systems; check at an auto parts store. If you can't find one, you may have to restore the bulbs.