O/T '01 F150 9.75 rear end noise

[dstryr]

Looking for a little help here...  The rear end in my '01 F150 has been making noise and shuddering lately when starting from a dead stop and turning either direction.  I figured it was time to change the gear lube so pulled the cover and drained the fluid.  Other than dark it looked fine, no metallic looking glitter when I set the drain pan in the sunlight.   I cleaned out the depression in the bottom of the case which trapped about a cup or so of gear oil and found nothing in the bottom so put it back together and added 4oz friction modifier with the 80w140 synthetic lube called for in the manual.   I took it for a 40 mile drive but the shuddering is still there, though significantly better.  Truck has 173,000 miles.  

What's going on in there?    

Thanks!