Intermittent left turn signal - 1999 Jeep Grand Cherokee

Generally, if a bulb has failed or is making only poor contact, the blink rate will increase since the standard blinker relay uses a thermal strip heated by current flow to the bulbs and when it is too low, it blinks rapidly and briefly.
You still have the front bulbs to check and maybe pulling the fuse for that circuit and plugging it back in might remove some corrosion from the blades.
The flasher relay is probably OK since this would affect both sides.