I finally saw it. The episode of Harvey Birdman: Attorney at Law, where Harvey represents Shaggy and Scooby arrested for... what else? Marijuana!
I've been waiting a long time to see this episode, and if it comes on again I highly recommend you catch it. (No pun intended.)
My favorite part? Right before the trial, Harvey is sitting at a bar:
Harvey: There are two lives that depend on me giving the performance of my life in court tomorrow.
Harvey: Hell, yeah! Let's get stinko!
Oh, and keep your eyes open for a cameo by George Jetson. Loves it.