your description of kickdown isn't really what a kickdown is. Kickdown refers to a downshift with maximum or strong acceleration when already driving at speed. Sounds like you are talking about accelerating from a stop. I'm thinking it has something to do with the driveshafts.
2008 Mercedes-Benz CLS550 Iridium Silver/Black Leather -- P2/321/881 (leasetraded 1/10)
2002 BMW 325i --Topaz Blue/Natural Brown Leather --SP/PP/HK/CWP/Bi-xenon (Sold 9/06)