Just sounds like a bad ground to me. Make sure the bulb and light sockets are clean where they touch. Clean up any corrosion you see on mating surfaces with some medium grit sandpaper. If that doesn't take care of the problem, get yourself a piece of wire and hold one end on the bulb socket and the other end on some bare body or frame steel (not rusty) while the blinker is supposed to be working. If it works then, you have a bad ground and will have to hook up a new one. Don't worry, you can't get electrocuted.
Gary Greenway - KD8NHC
08 4 Door Rubicon
Member of Two Trackers 4wd Club
Great Lakes Four Wheel Drive Association
United Four Wheel Drive Associations