A Sawsall with a good metal cutting blade works well for making a slit in an internal, male, fitting stuck inside a female fitting and also vice versa. With the Sawsall though you can put the blade inside the whole pipe. Once you make the slit on the inside or outside you should be able to peel or pry the piece off. Autozone lends out an exhaust pipe expander and also sells fittings for you to go from pipe to pipe. Two nights ago I had to fix my exhaust at the same place as My s pipe broke off in the muffler. Both relatively new. I had a hard time getting the peice out of the muffler but then bought a 2" ID to 1 7/8" OD fitting to connect the system back together.