They don't fail very often. Only if when they are replaced they are either damaged when pressing in, or if the rear knuckle is damaged when the bearing fails, and someone tries to put a new bearing in it. If the bore of the knuckle becomes too large from a damaged bearing, any new bearing placed in there will fail shortly.
they dont usually go out. it might be easier and cheaper the find your rear sub-frame at a junk yard and just swap the entire sub-fram. all you have then is 4 bolts, the drive shaft and brake lines.
that would be to tempting to do an entire irs rebuild. might just take it to ford...don't know yet. but, thanks for that info. though. more time i didn't want to take.