The 1969 song is performed at 7:08, from this concert:
The uncle is Uncle Sam, and the song is about draft dodging during the Vietnam War. Evading conscription, intentionally, sometimes required draftees to fake an inability to serve. Others had the resources to seek their freedom in the courts. Around 30,000 evaders left the country and this song is about leaving for Vancouver to avoid military service. It was not against the law to be a draft dodger in Canada. But those returning home could face criminal penalties (until pardons were implemented by President Jimmy Carter).
If you’re wondering, the Flying Burrito Brothers were formed by two members from the Byrds, Gram Parsons and Chris Hillman. They are considered an early country rock group. The album, Gilded Palace of Sin, was not a commercial success, but highly influential.