Firing steel from a Full Choke is a prescription for the problem you describe.
Clemson
So is a plain old fashioned barrel obstruction, i.e., water, mud, snow... Choke tubes, being a little harder than the barrel, don't stretch well, so anything that puts stress on the tube is likely to crack it. Most folks around these parts prefer a much more open choke for steel shot. Something on the lines of IC or Cyl.
Clemsons correct, cut the barrel back to the next rib post and either have tubes reinstalled, or, take it out and try it wide open with your steel shot.