What has changed? Is it in more or less shade that in previous years? Is it getting crowded by other roots? Tree peonies can grow for hundreds of years, shouldn't decline without a reason. Do you know the variety? Some tree peonies can be root divided like a herbaceous peony, some have more of a tap root and root division doesn't work that well. The most sure way is probably to layer a lower branch. Find one that looks healthy and remove leaves from a node, sprinkle rooting hormone on it and peg it to the ground. Leave it there until roots form, could happen same year, possibly longer. That's the way I'd start with but if you have experience grafting plants that would be quicker and easier.
Once you have new plants propagated I'd probably try a rejuvenating prune on the original plant. Remove all old wood and see if new stems emerge the following spring. I'd wait until I knew I had a new plant before taking the risk though.
