Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history, I can not tell you what the exact cause of your symptoms is. However I will try to provide you with some useful information.
Buttock and hip pain can have many potential causes. Non-neurologic causes include vascular disease (narrowing of the arteries to the legs, if it occurs in the aora for example, can lead to pain in the legs, particularly with exertion), arthritis, tendinitis, muscular. Neurologic causes include radiculopathy (commonly known as a pinched nerve, or pressure on the nerves as they exit the spine, due to arthritis, herniated disc, or narrowing of the spinal canal). Neuropathy, post-viral or otherwise, is more likely to affect the nerves distally (in the feet first before ascending all the way up to the hip), though proximal neuropathies can occur; vitamin B6 toxicity (overdose) can lead to this type, as can some post-infectious disorders (such as what is termed AIDP or Guillan Barre). These however are unlikely to cause persistent pain restricted mainly to the hip and buttocks. Some of these can be investigated with an MRI of the spine, others require an EMG/NCS test which tests how nerves conduct electricity and how muscles respond.
The pyriformis is a muscle in the buttock, between which the sciatic nerve passes to the leg. Some physicians believe that if this muscle is enlarged or otherwise irritates the sciatic nerve, buttock pain with pain down the back of the leg can occur. This area is controversial in neurology.
The peroneal nerve is a branch of the sciatic nerve; it starts right above the knee and passes along the lateral aspect of the leg. The peroneal nerve is not a commonly biopsied nerve; and peroneal nerve biopsy in the leg would be unlikely to cause buttock pain.
Physical therapy, if recommended by your examining physician, is often found to be very helpful for many different types of pains. Continued followup with your neurologist is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.