Is it still under the 5/60,000 power train?
Look on the left side of the axle and see if there is any moisture on the tube, if so take it back to them and tell him to look up the TSB 12-8-15, it lets fluid out and plugs sometimes causing the pinion seal to leak.
2013 V6 MCoA, Performance Package, Security Package, Comfort Package and Electronic Package.
Mods OEM Backup Camera, BBK Shorties, FRPP Cut and Clamp X-Pipe, Borla Touring A/B's, Airaid CAI, Axle Xchange 2 piece aluminum Driveshaft, Koni Sport Shocks and Struts, Vogtland V6 Leveling springs, Ford Racing 302s wheels wrapped with Continental 275/40 19's, JBL GTO 8628 6X8 in all 4 corners. 2013 F 150 FX2 With a 6.2 Boss