--implies a neuropathy, but possibly not of the kinds we most often talk about here; from my prespective this does not look like a neuropathy secondary to a systemic body-wide condition (i.e., diabetes, toxin, deficiency) but rather one caused by mechanical forces, such as nerve compression.

Not that underlying conditions might not make the symptoms of nerve compression worse (see "double crush hypothesis"), but given that positional changes and pressure changes seem to have considerable effects on the degree of symptoms, this sounds like there may be compression on nerves that go down the legs. The trick is to find out where it's coming from.

It is certainly possible that there is something going on in the lumbar spine that might be contributing to this, or even within the lower sacral spine nerve roots. From what you describe, though, I'd take a look at the sciatic nerve and its branches as well. It is actually quite easy to compress nerves in both the lower and upper leg regions--there are a lot of choke points at which pressure can be applied, even by overdeveloped muscle and fascia (see "piriformis syndrome").

Some articles to look at:

http://emedicine.medscape.com/article/1234809-overview

http://www.emedicinehealth.com/sciatica/article_em.htm

http://emedicine.medscape.com/article/87545-overview