The car has a 4.7 L V8, 69,000 miles and while going up a mountain in VA we could feel a “Klunk” under the driver and passenger seats. It felt like we went over a bump in the road, but that wasn’t the case. It only happened when the engine was under a heavy load. Now several month later it is starting to happen on flat roads. It only occurs after driving for about an hour, it will do it for a few times and then go away. The dealer can’t recreate the problem but I’m concerned it may be something serious. Is it a drive train or transmission problem?
have you checked the level of transmisson fluid? It is an automatic yes? This sounds like an automatic transmission that needs servicing. As the problem is getting worse, and not better, take it to a reputable independent transmission service, or if you have the extended warranty, take it to the dealer.