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.

I don't believe there is one, but try the Castlemaine Rod Shop (03 5472 2853) as they do a lot of that sort of thing and might have something they could help you with. If they don’t you could have the conversion done by a Vicroads accredited engineer, but it would an expensive exercise and I would think it would be more economical to buy a replacement diesel engine for your van.

If it were the actuator that has failed again I would think you had a strong claim for a replacement. The part that was fitted has a warranty, and if it has failed, the warranty on the actuator should cover the replacement.