Just replaced all my stock indicators with LEDs from Cycle Gear.  Below 45mph they work just fine but as soon as I go over 45 they won't work at all.  Fix?

you mean the turn signals or do you mean the 'dashboard' lights?

The actual indicator lights.

Check the rectifier/regulator.  If that is ok, probably need an electronic relay.

Check your DC voltage. It may be a bad regulator. At high RPM, the voltage may go too high and the LEDs cut out for protection. Connect an incandescent light in parallel with the LED and watch it carefully. Does its brightness change? Use your VOM (volt ohm meter). Stick it somewhere and watch the voltage at low and high RPM. That will really tell you what is happening. I think it all comes back to bad voltage regulation.
What happens if you just rev the engine really hard? Same thing? If not, then I have no idea why speed makes a difference. RPM would change the voltage, not MPH.

Where can I find the regulator?  Is that located right next to the relay?

And it isn't RPM dependent, just MPH which is why I'm stumped.  I don't want to but if I have to I'll switch back to bulbs rather than be without turn signals.

Check around the battery box.

It's dependent on the speed, NOT the rpm's? That's weird.