I was also born in Polk County. My 3rd great-grandfather, Isham Russell Lilley served in the civil war. I've added his information below and a link to the cemetery transcription from where he is buried. I've been able to locate a little bit of information on him, but I'm not sure whether he is related to your Lilley's or not. I have a resource at home that I can check.

Lilley, I. Russell- b-June 7, 1830 d-May 30,1899

PVT Co D Flournoys REG 16 Texas INF CSA (dod May 13 on MM)

I hope this helps!