I've found the numbers for three different plates. As I have an '02 z-71 i don't know which is standard equipment and which is part of the z-71 option.
The first plate is located near the front of the engine and sits at an angle, higher in front than back and has grooves in it. part # 15203366, "shield, engine", 2007 thru 2011 retails for $27.22 (catalog section 00, engine)
The second plate is under the oil pan, is mostly flat, part # 15843753, "plate, oil pan skid" 2007 thru 2011, $42.56 (catalog section 01, cooling system-grill-oil system)
The third plate is under the transfer case, has some grooves/contours in it, part # 15049809, "shield, transfer case ", 1999? thru 2010, $36.22. (catalog section 04, automatic transmission)
All use the same bolts, part # 11518341, M10 x 1.5 x 25, 14 thread, 28 O.D. (because of integral washer), $ 6.79 each, 4 bolts needed for each plate.
Hope this helps. all info is from GM's Electronic parts catalog. If your dealership guys couldn't find the info, then I would try another dealer. I found this stuff out and I don't even work for any dealer or parts retailer.
later. paul.