Noticed the beginnings of the typical "wah wah wah" noise of failing bearings soon after my mother-in-law purchased her new '03 model LS. I inherited it from her with around 35K miles a couple of years ago. As the noise got progressively worse, I knew something had to be done. Long story short... both rear axles had to be replaced within a year of each other.
My research shows that a batch of poorly heat-treated axles were installed in the era around '03 on Marquis' and Crown Vics, but Ford never owned up to their screw up, so any fix is on the owner.
I inspected both axles at time of removal, and both showed fretting in the bearing contact path. The damage had not progressed so far as to be dangerous, but the noise and fretting showed that, if left to continue, total failure of axle or bearing would occur if not replaced.
If you suspect this problem, have your guy put it on the rack, with rear wheels free to spin, then put car in gear and listen to rear end sounds.