I haven't found anything definitive yet, but according to my research they moved to San Bernardino in 1978, so that helps at least approximately date the age of the song.

He only produced 2 albums under his name after that point. He didn't have writing credits for any of the songs on The Bramble & The Rose, but on Endangered Species he wrote
- Dusty Desert Wind
- Still Got That Look
- Brand New Jole Blon
- Linda's Out There On Her Own
- Bayou Blues
- Whiskey And Cocaine

Albums from Mary McCaslin after 1978 with self-written (either her or Jim) songs include:
Sunny California (1979)
- Sunny California
- The Emigrant Song
- Old Man From Missouri
- Dust Devils
- The California Zephyr
Blue Ridge Epitaph (1981)
- Time To Wonder
- Set Me Free
- Jamie
A Life And Time (1981)
- Northfield
- The Band Of Jesse James
- A Life And Time
- Santana Song

As well as most of Broken Promises and all of Better Late Than Never

Do any of those ring a bell? Several songs from Sunny California seem to have pretty promising titles but I haven't listened to all of them yet.