Pogo, sorry to hear about your divorce. By 401K I assume you mean TSP. Have a look at this pub on the TSP website:
https://www.tsp.gov/PDF/formspubs/tspbk11.pdf
Are you retired or nearing retirement? If nearing retirement, you may need copies of your divorce papers to submit with your retirement paper work. I just retired from the PO, and there were questions about former spouses (mine were before I started with the PO), so I wasn't too concerned with them to remember what they said.
But as was mentioned by tsp talk, definitely calls for an attorney, especially one familiar with federal service.
At least the two of you are agreeable on not messing with each others retirements which will make things easier.
All the best to you.