Mechanical fan on 05 isn't it? I know you said you changed the fan clutch but that is tell tale sign of fan clutch failure, unless you are overheating.
Are revs going up with fan noise? Should be.
GE Nighthawks for life! I've had an amazing history with Nighthawks.
http://www.rockauto.com/en/moreinfo.php?pk=2548096&cc=1385355&jsn=579
or
http://www.rockauto.com/en/moreinfo.php?pk=2548186&cc=1385355&jsn=580
http://www.revbase.com/BBBMotor/TSb/DownloadPdf?id=131011
If it is the differential, which it probably is, the above is the TSB. Get the bearings replaced and you'll be on your way. A reputable differential shop can show you the issue when disconnected.
A kit does exist.
The parts are hundreds of dollars. Ford quoted me $2100 to do the job this year.
I instead took it to a local rear end shop who installed a full "Master Bearing" kit in my rear end, reusing the ring, pinion, and carrier/carrier internals. Got it fixed for $450...
Is it just seeping? High mileage oil can cure that problem.
If that doesn't work, Bar's Leaks is great stuff I've used personally and have friends that have used. Use exactly as directions state and every time in my experience it does the job well.