I am not sure how the pinion depth is set on a Mustang, but on a GM it is done via a crush washer. On a GM diff, if you remove the yoke you almost always have to replace the crush washer and redo the pinion depth setting. I have seen several GM diff's get nuked after someone has remove/replaced the pinion yoke without replacing the crush washer and redoing the pinion depth. They simply crank on the pinion yoke and it pulls the pinion depth out of alignment and it screws up the ring and pinion mesh, resulting in a howling diff that eventually fails.

If you have not properly setup the backlash on a diff before, it may be worth the time and $$$ to take it to a shop to do it right. I consider myself a fairly competent weekend warrior mechanic, but screwing around with the diff backlash is something I will not touch with a 10 foot pole. I have learned that lesson the hard way already.