The speaker begins with a worry about language. However carefully they sign and swear, promising word by word, something keeps changing. Nothing they say stays quite the same once it is said, so even firm commitments feel unreliable. The title, "Shifting ground," fits that unease.
Then the poem moves down and inward. The speaker goes from "one private doubt" to "a private land," as if doubt were a slope leading into a whole country of its own. It's a lonely place, and nobody else seems to be there.
The last image is surprising and oddly calming. The speaker buys a pig and checks her pulse like a doctor. After all that uncertainty, this is a physical, practical act. A living heartbeat is something you can actually feel. The mood is thoughtful and a little uneasy, but the ending suggests the speaker finds some steadiness in touch and in the real, ordinary world.