Is MIL local? This would be better if you could do it in person.
OWN the previous stuff at the outset of the call. Use the word "respect" liberally -- "Out of respect for you, I wanted to give you a heads up, and let you know that I'm about to do something very painful, but I feel it needs to be done . . ." "Even though we've had our differences, I've always respected you, and I respect your position as the mother of my wife, and so what I'm about to tell you is very painful for me . . ."
etc.
Do not say ANYTHING that you don't want to get back to your wife.
