Grumio’s speech changes from when he is addressing his fellow servants to when he is speaking to Petruchio. What is ironic about this change?in...

Though I highly doubt this is pertinent to you anymore as you posted this a year ago:

Grumio speech when addressing his fellow servants is marked with the more formal "thou" and "thee", which suggested a much more distant and impersonal relationship between the speaker and the addressee. When he speaks with Petruchio, he speech changes to the more acquainted "you" and "yours", which is much more conversational and intimate.

The irony has already stated itself. One would believe he would use the former with his superior and his master, and the ladder in chatting with his compatriots and fellows, but he does not. Shakespeare does this for mainly two reasons: to heighten the ironic wit and sarcasm of Grumio's character, and to imply a long-standing and fairly secure relationship with Petruchio, and placing himself as a sort of first man in the hierarchy of servitude.

The best example of this change is comparing Act II, Scene 1 and Act IV, scene 1, wherein you'll find Grumio conversing with Petruchio (the master) and Curtis (fellow servant) respectively.