It’s possible – although pretty unlikely - that a modern car still uses bronze bushings somewhere in the differential or drive-axle assembly. The material is likely to be bronze or phosphor-bronze.

I’m tipping you’ve performed a differential-oil drain and found a few metallic-looking bits in the bottom of the rain pan. This is never a good thing to find, as it suggests that something is wearing inside the diff and shedding these filings. It’s actually not abnormal for a few tiny fragments to appear in the oil, but if the bottom of the drain pan glitters like you’re panning for gold, then you have a potential problem.

Even though the fragments might look yellowish, they could have started out silver (like normal bearing material) but have become discoloured due to time and heat in the environment in which they operate. So, just because they look like bronze or brass, they could be made from any type of bearing material. At which point you need to investigate further.

There’s every chance this is caused by a small amount of wear and slack in one of the joints in the driveline. If that’s the case, those joints and couplings need to be checked and the worn one(s) replaced, as continuing to drive as it is could see the driveline fail spectacularly at speed.

The reason you feel the jolt on the flat and not when you’re parked on a hill is that the driveline is relaxed on the flat, leaving a little slack for the thump to occur. When parked downhill, gravity has caused the driveline to take up that slack (plus your foot is probably on the brake, preventing the car rocking or moving at all) so the thump can’t occur and, therefore, isn’t felt.

There doesn't seem to be a huge number of similar complaints to yours, Len, but that's pretty cold comfort. You could have a problem with the transmission itself or the electronics that control it. Either way, it could be that the transmission is stuck in high gear as a form of limp-home mode to avoid doing any further damage to itself.

The first step is to have the vehicle scanned for fault codes. Sometimes, a fault will be logged that seems to have nothing to do with the symptom, but can still cause the havoc you're experiencing. Try a transmission specialist first, as it's a fair bet they will have seen this exact problem once or twice before.