Peggy answers the phone during dinner, and when she hangs up, Hank asks who it was. Peggy said it was her mother. Hank replied "wow, what's it been, 20 years"? But just back in season 4, Peggy and Hank both talked to her mother on the phone, they were at the airport trying to get to her house for Thanksgiving. In Season 3, Ep. 4, Pregnant Paws (1998), Hank replaces Peggy's mom's number on the speed dial, indicating she's regularly called, and later Peggy tries to call her to stay with her. So it clearly hasn't been 20 years since Peggy spoke to her mother.
While driving the cattle through the main street of what the viewer is told is a town in Montana, Peggy quotes from the "Osage County charter." There are no counties in Montana that start with an O. There are Osage Counties in Kansas, Oklahoma and Texas.
The Hills travel to Montana where Peggy attempts to make amends with her estranged rancher mother; in Season 2, Ep. 14 ("I Remember Mono"), Peggy & Hank attend the same school in Arlen and Peggy's Mother is clearly seen looking like Peggy. In "A River Runs Through It" Peggy's Mother chastises Peggy for leaving the ranch (clearly in Montana) & not marrying "the boy from across the way" that she knew in school.