How disabled can you be? Any specific rules on pain meds?

Most companies won't take a chance on you. It might not be legal to discriminate against a worker with prior health problems so they'll find another excuse but the bottom line is that you present too much risk.
There are enough healthy non medicating drivers available to hire one with a known health problem.
Workman's Comp is already very expensive and additional claims will raise the rates. Most trucking companies don't need the cost or aggravation.
Sorry, but that's the way it is.