I had a tranny shop tell me that it would be $600-$700 to do that, and
suggested it as "preventive maintenance". Needless to say I don't have
any work done at that shop. I'm learning now, after not having to have
my truck in the shop for 8 years, that it is sometimes not a bad idea
to go to Ford. The nice thing is that it is easy to call them and ask
how much to replace the pinion bearing. They should be able to tell
you parts, labor, and tax.

-Joe