AutoNation Ford Katy

My truck is a 2006 F150 with only 34000 miles on it. I started hearing a whirring noise from the rear axle, knowing it was possibly a bearing of some type? I take it in, explain to the service writer what I've been experiencing, left the vehicle and was on my way. A day or two later he called saying the tech diagnosed the issue as the driveshaft and should be replaced to the cost of $1350...I approved, and they ordered the driveshaft and it comes in a week later, then says they took the old one out and it looked fine, then looked around and noticed the yoke to the rear differential had some unusual play, so without opening and examining the rear differential, the tech "diagnosed" a COMPLETE REAR DIFFERENTIAL REBUILD to the tune of $3045.00!!!! I found that unacceptable considering the suspicious circumstances of the "diagnosis". I declined, paid the diagnostic fee, though I feel it was undeserved, and took my vehicle to my longtime mechanic for a second opinion, who was able to diagnose a simple rear diff pinion bearing R&R to a total repair cost of only $869. Way better than an unneeded rear differential for $2100 more! This wasn't a real mechanic, but just a removal and replacement specialist who has no real diagnostic skills whatsoever. Fun fact: a dealership's service department only needs one ASE Certified technician to claim the entire facility as ASE certified. I will NEVER take my truck back to a dealership for service unless it is under warranty, period.