[zipzip]

Seems to me that if it only develops the problem when it's at temperature, it's either lube related (hot vs. cold viscosity) or some sort of alignment that goes out of whack when everything's hot. Driveshaft isn't bottoming on the trans at temp? Some sort of alignment issue on the shift housing? Have you tried to idle the engine up to temp without driving and then see if it repeats? Or is it only when the trans gets up to temp? Jack up the rear wheels and pull a Ferris Beuller (sp?) to warm up the whole drivetrain. Just don't do EVERYTHING Ferris did...

Dirty Harry might be onto it too - a worn bearing that is pretty stable cold, but wobbly-wobbly at temp...