"PN" is peripheral neuropathy, which refers to any damage to nerves that are not in the brain or spinal cord.

There are over 200 causes of peripheral neuropathy, ranging from the obvious, such as mechanical damage or compression (which would be my first supposition in your case, given you first noticed in while driving, which is not good for many of the nerves in the back of the thigh, especially the sciatic branches, although hopefully you will give us more information and history) to many far more obscure etiologies.

If you haven't already, come on over to our neuropathy forum--lots of expertise over there:

http://neurotalk.psychcentral.com/forum20.html