Most automotive flashers are a thermal type. They need high current draw to heat up, which then breaks the circuit by way of a bi-metalic strip. When the bulb burns out there is less current draw and the blinker blinks fast. This is an unintentional benefit in that it alerts you there's an issue with your turn signal.

LEDs consume WAY less current than incandescent so the thermal flasher can't work. To fix this you need an electronic flasher module.

TL;DR: buy an electronic flasher module

EDIT: a part. (never tried, not affiliated)

https://www.amazon.com/1998-2011-Ford-Ranger-Signal-Flasher/dp/B00KQUYW8A