Don't you just hate to spend the money on ribs, season them up, cook them for hours and then have tough meat?
One quick tip for having tender barbecue ribs is to remove the membrane. That's the shiny white coating on the bottom side of the ribs.
It would certainly be nice if the rib membrane was removed at the butcher shop, and you can ask for it to be removed (and pay more), but it's not difficult to do it yourself.
Basically you just get the membrane edge peeled up and then just keep easing the membrane away from the meat. It may or may not come off in one piece. If it breaks, just start a new peeling edge and keeping work toward the bottom of the rack.
For those you learn better by seeing a technique done, this You Tube video from the BBQ Pit Boys is quite helpful.