There are few television series that do musical episodes. Even Stevens had one with great reviews while 7th Heaven had one with bad reviews.
It's Garry Shandling's Show spoofed an Afterschool Special with a musical theme to it. Everyone had a singing number (even Garry & Leonard, a little) and the songs were funny. My favorite part were the singing plumbers who were fixing Garry's toilet. When they sang "We even tighten the seat!", I had to pause the vcr because my brother laughed over that part & had to recover.
Great episode! It's the 80's version of High School Musical, somewhat.