The clunk from the rear on acceleration turned out to be a drivers' side motor mount. I elected not to pay $500 to replace both but to do the bad one myself. At O'Reilly's, an Anchor mount (Chinese) cost $35. Took a few hours but I never had to loosen the passenger side. Drivers' side is in tension upon acceleration while passengers' side is compressed and should never fail. Shop refused to split the $500.

Clean engine top and bottom at carwash: remove clipped on airbox top; snap off motor cover to better observe potential stretching or interference during jacking; remove 2 bolts and U-nuts holding bottom of fan shroud; LF wheel off; jackstands; pull wheel liner; remove heat shield (3 10mm nuts); unbolt top motor mount nut (18mm) and remove heat shield; securely locate hydraulic jack under LF engine corner with 4"x4"x12" vertical push block to lift front corner of crankpan (I clamped it to anti-sway bar with a couple carpenters' clamps to make sure it stayed in place; jack till the top of the mount retention bolt clears the bottom of the engine bracket; slip a stable length of 4x4 betwen the anti-sway bar and the crankpan to protect your fingers if your engine jack system fails; remove bottom motor mount nuts (16mm/5/8") using a long extension; the back bolt takes a deep socket or short extension and then a universal ahead of the long extension; pull out the failed mount; grind 1/16th inch off the top bolt on the new mount for ease of fitment and install with "FRONT" embossment towards truck front; motor mount is not centered but fits in forward holes on frame bracket: put it all back together; and, no more clunk (which we all thought was a worn transfer case chain or failing differential.)

Note: jacking cannot overstretch the bolted down passengers' side as mounts have a built in stop (or your engine would have tried to exit your hood already!).