This sounds like a rear differential problem. The differential is designed to allow the inside and outside wheel of that axle to rotate at different speeds (which they need to do as the inside wheel describes a smaller arc than the outside wheel in a corner). If something goes wrong inside the differential, that function can be lost and the wheels try to turn at the same speed in corners. When that happens, the rear end will skip and jump about as the two rear wheels effectively fight each other.

Limited-slip differentials (LSD) often have this problem when the clutches that control the amount of lock-up in the differential fail or become worn. At that point, similar jumping and clunking noises can be felt and heard. In some cases, a limited-slip differential can be restored to health with a change of fluid and perhaps an additive, but in other cases, the unit must be rebuilt.

The rear differential in your D-Max should be the limited-slip type, but around this time, Isuzu made the LSD an option, so it would be worth checking. If it’s an LSD, a fluid change would be the first step towards sorting this problem.

Remapping an engine – done right – can provide you with more performance. But there’s also often a price to be paid in terms of reduced fuel economy and increased tailpipe emissions. Typically, there are workshops that are very good at this and others that aren’t so expert at extracting more performance.

But before you go down that route, it sounds like there’s something wrong with your vehicle in its current state. Fundamentally, a 2013 Isuzu D-Max should be able to haul a van of that mass, comfortably at the legal speed limit. That said, that’s a fairly hefty caravan and experience shows that many owners of similar rigs find that about 90km/h is a more comfortable speed for highway travel. So maybe you need to lower your expectations a little.

A full service and tune-up would be the first step in improving the towing situation, by returning all the engine’s settings to their stock position. Modern turbo-diesels can also suffer a fall-off in performance over time as the fuel system (pumps and injectors) wear, the diesel particulate filter becomes full of soot and the intake system becomes clogged with a mixture of soot and oil from the vehicle’s emission-control system. Those are the things we’d check first before shelling out for a remapped engine computer.

D-Max models before MY 17 do not have a DPF. The later Euro 5 compliant 4JJ1-TC engine does have a DPF filter.