Dear Kay, I inquired some time ago about this same issue. Here is a response I got:
You are not alone in your confusion about this supposed Decker Township. I have a copy of an 1820 census page, taken from microfilm on file at the Bedford Library, on which I wrote the reference Decker Township. The individual I was researching was Ezekiel Blackwell who was one of several who donated land in 1818 for the new county seat of Palistine. Ezekiel operated the ferry on White River there in that early town, and for several years after the county seat was moved to Bedford.
Research of the early county history shows that while Lawrence was still a part of Orange County, this area north of the White River was organized as Leatherwood Township. Upon the creation of Lawrence from Orange in 1818, the area around Palistine and north of the river was named Shawswick Township, and remains so to this day. Unanswered is, was there ever a Decker Township, or was this just a census catalog error?"
This doesn't answer the question but, might give you a little insight. My 3rd great-grandfather, Nathan Melton, is listed in the 1820 census in Decker township also. He & his family lived in Perry township for years. Perry township is the furthermost northwestern township in the county & borders Greene County to the west & Monroe County to the north. I tend to believe that Decker is Perry township but, I really don't know and it seems that no one does.