2002 Mountaineer rear end noise

vehicle is 2 wheel drive,4.0L v6. developed a rear end whine that varied pitch with change in speed, most noticible at 30 to 50 mph.Dealer replaced ring gear and pinion,under warranty at 35,000 miles,now at less than 4,000 more miles the noise is back,when picked up from previous repair the tech. who did the work stated he did not check amount of lash...I am taking the vehicle back....but am interested in your comments before I do so...

common problem - ford has released an updated rear end for this problem - but besides the noise the rear ends are fine - it's annoying I know but they don't seem to be wearing out any faster than the quiet ones

noise was caused by wheel bearings on the passenger side rear axle..all has been well for the last 7,000 miles.