Goldfinch,
There is a company in Watsonville CA owned by some friend of mine that builds custom recumbent bikes and trikes. Because they are custom fit, they may be able to build a two wheel recumbent that would fit your stature. I know some of them can get pretty expensive, but they do have some for around $2000. I personally have a hernitated L4 from about 10 years back, and, while i don't have muscle spasms it does give me trouble from time to time. The relaxed seating position of the recumbent bikes does make it a lot easier to enjoy a long ride. If you want to check them out their web page is www.easyracers.com

Hope you find a solution that works for you.

Brandon