Some 2010 Ford Fusion, Taurus, Edge and Flex; Lincoln MKS, MKZ, MKT and MKX; and Mercury Milan models with AWD systems may produce a shudder, chatter or vibration from the driveline when making tight turns. They may also emit a thumping or clunking sound on light acceleration or under a heavy load while driving at speeds under 40 mph. According to Ford, one possible cause for all these symptoms is a defective power transfer unit (PTU).
To home in on the diagnosis, start by getting the vehicle on a lift. With the transmission in Park, see if you can rotate the driveshaft by hand using a strap wrench. If you can, the PTU is damaged. Another method is to set the parking brake, start the engine and put the transmission in gear. If the front wheels turn but the driveshaft doesn't the PTU has seen better days. New-design, heftier units are now available to address the concerns.
Here are the part numbers for the upgraded PTUs: Edge, MKS, MKX, Taurus (AT4Z-7251-A); Flex, MKT (AA8Z-7251-B); Fusion, Milan, MKZ w/AWF21 trans. (7E5Z-7251-F); Fusion, Milan, MKZ w/6F35 trans. (9E5Z-7251-AG).