What's causing this high frequency ripple on the output of my LM317?
Make sure that there is a small ceramic decoupling capacitor at the supply pin(s) of the microcontroller. Something 100nF - 1uF should be OK. (The 470uF cannot deal very well with HF noise).

How is the grounding on the microcontroller side? Is there a large ground plane?

Is the circuit return earth-referenced? Is the scope connected to the same earth reference?

If you don't have a spring-clip for the ground, and don't mind abusing a probe, you can solder short pieces of solid wire from the supply rail to the probe tip and ground ring (under the probe tip) which will cut down on CM noise pickup dramatically.

In general, most ripple measurements in the switching power world are done with short probes (or direct coax) with 100nF and 10uF ceramic / tantalum caps shunting the probe and the scope set @ 20MHz bandwidth limit. (CM noise &lt;> PARD)