Not all epidurals are the same. Epidurals in laboring patients can cause leg weakness since they target pelvic innervation, although at my hospital these patients aren't absolutely prohibited to ambulate, it's depends on the patient. There are some studies that show it's safe to ambulate laboring patients with epidurals and may even have some benefits.
In patients with thoracic coverage typically for a thoracotomy, the epidural shouldn't prohibit ambulation and long as the coverage is appropriate (T3-T6 dermatomes for instance). I think many Nurses just think "epidural=bedrest" without really thinking it through. Thoracotomies often have epidurals since it is an extremely painful post-op course, yet these same patients are often at high risk for many of the complications that can result from not ambulating, so the Nurse's you work with really need to get over the misconceptions they have about epidurals and ambulating.