My 2004 Silverado SS has 343000 miles on it and has been well maintained but driven very hard.  The only codes are bank 1&2 o2 sensors and a fuel canister relay for emissions.  The truck had the transfer case replaced at 300000 miles.  The truck has original engine and transmission.  It still runs very strong but has recently at high rpm at the shift point the engine looses pull and motor sounds rickity and won't shift until I let off the gas just a little.  If I put it in tow haul mode it will shift fine (I call it sport mode because the truck runs completely different).  What would cause this problem.  The truck has no upgrades and has been the most reliable vehicle ever built.