I know exactly how you feel – If you go back about three months or so I was asking a similar question on this forum – lots of back pain, particularly lower but sometimes upper. What I hadn't noted was that I also occasionally had wrist pain. I was also looking at suspensions seat posts and maybe a budget FS.
(Touch wood) I seem to have just about eliminated the problem – including 5 laps at Mayhem without a single tweek from the back (wasn't that a fun weekend!?)
My cure seems to have been a combination of learning to relax my shoulders (relax hand grip and focus on relaxing shoulders), working on my core (using the power muscles less, so putting less strain on the lower back) and adjusting my riding style – my centre of balance was often way too far fwd!
Some massive benefits I have noticed (thanks to the advice on this forum) are that the back pain has just about gone (and when I start getting it I know why and focus on relaxing), I have saved a load of cash I couldn't really afford anyway , my riding has improved (!) and I am actually looking forward to more racing rather than worrying how long my back will last…
I have no doubt a FS will help but there is a lot you can do without splashing out cash… Good luck!