I'm a little late but will put my 2 cents in. The cure for back pain on my 03 FXDL was a rider backrest and FXLR bars. Prior to this 12 1/2 works shocks helped as well.

 

I had a 2007 low rider that I put a sundowner seat and progressive suspension 418 shocks on, totally changed the bike for the better and much easier on my back. The shocks made the ride much smoother and much improved handling over the stock shocks. I just got a new fat bob, first thing I did is order a new seat, (saddlemen's this time) and 418 series shocks for it. They're pricey but well worth it in my opinion. I only weigh 160lbs but the shocks come in standard and heavy duty versions.