rear end axle

I have a 96 grand marquis ls with the digital dash and I would like to swap out the 2.73s for 3.55s how do I reprogram the speedometer?

So i own a 2009 Mercury Grand Marquis limited edition, im planning on swapping out the rear axle (2.73) for a posi lock (3.73?) Can i just remove my axle and swap in a new one or are there other things i need to add or remove...