That the shudder only occurs in fifth gear is, perhaps, the big clue here. If the problem was an engine mount or drive-shaft or suspension problem, the shudder would appear at those same engine revs or road speed, regardless of what gear the car had selected. So maybe you need to dig a bit deeper into the transmission to see if there’s a problem with the torque converter, the way the converter lock-up is working or even a problem with the gearset itself that offers up fifth gear. This is, of course, if the vehicle is an automatic (which you haven’t disclosed).

If it’s a manual, then fifth gear is the overdrive ratio for that gearbox and some Colorados were fitted with five-speed manuals that had a problem with the thrust washer on fifth gear. This washer could fracture if the vehicle was used for towing heavy loads. When the washer finally failed, the vehicle could jump out of fifth gear and actually lock up the rear wheels. Back in the day, Holden was actually replacing the entire gearbox if the vehicle was one of the affected ones. It would be worth checking with a Holden service centre to see if your vehicle is from that batch.

Most glow-plug systems in diesel cars work on a timer system. The manufacturer will have worked out that X seconds of current will heat the plugs to a temperature sufficient for easy starting, so that X-seconds period is where the timer will be set.

What happens, though, is that glow plugs can lose their efficiency over time (and with 233,000km on board, your car is a candidate for that to have happened). As that efficiency is lost, they don't produce the same amount of heat as they used to and, therefore, the timer isn't giving them long enough to heat the combustion chamber sufficiently. When that happens, the engine becomes hard to start. The other possibility is that the timer itself is faulty and isn't giving the plugs power for long enough. The fact that your engine starts easily after the first cold start of the day, suggests the glow-plugs are, indeed, at fault here.

The other possibility is that the car's electrical system is not sending enough voltage to the glow plugs to make them work properly. This can be verified quickly and easily with a voltmeter. This method can also verify the length of time the timer is supplying power to the plugs.

In vehicles where the timer has already shut off the power to the glow-plugs before enough heat has been generated, the solution is often a new set of glow-plugs. The good news is that these aren't expensive, nor are they difficult to change if you know what you're doing. The caveat is if you aren't sure of what you're doing, at which point the fuel system on a common-rial diesel injection system can be very dangerous to tamper with due to the super-high pressures involved.

A minor increase in noise when shifting to four-wheel-drive is quite a normal thing to happen. By selecting four-wheel-drive, you’re suddenly engaging the transfer-case output shaft, the front differential and the front driveshafts. So, fundamentally, there’s a lot more mechanical stuff happening, and that’s probably what you’re hearing.

But the change in noise shouldn’t be a huge one, nor should it involve a high-volume noise of any kind. If there’s something screeching, grinding or knocking when the vehicle is in four-wheel-drive, then there’s probably something wrong with the driveline. If the noises are loud or nasty, then you need to have the vehicle inspected to find the cause and fix it before it causes more damage.