Hi Steve,
I have a very bad lower back from an accident i had years ago, and my lower two disc rub together causing immense pain at times, to teh point i can no longer straighten back up when it happens, very embarrasing.
Ive found that literally ANY form of free weight work will improve the lower core stability and protect your spine, in fact, since stating training some 7 months ago, touch wood
(And i just did) i have had NO pain at all.
There are definately some exercizes purely to improve the core strength, but im unsure if thats ALL you want to do...