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An unusual rumbling from below

I live in Ethiopia and drive a '93 Nissan Pathfinder Diesel, manual transmission (not sold in the States). I haven’t had too many problems with it, but recently I notice that when I drive between 60 and 80 kph there is a shaking (similar to the feel when you drive over the grooved pavement which marks the side of a highway in the States) that seems to come from the rear center. The shaking can last only one second, or as long as 5 seconds. It happens when I drive uphill, downhill, turn or am in the straight. Should I be concerned about this? thanks!

How are your tires (first) and front shocks or struts? The front tires are the most likely culprits, especially since the shaking happens during a range of speed but also uphill and downhill and level. Check for even and correct tire pressure, wear, signs of age such as cracking and splitting side-walls and treads, etc.
Are the front tires the same brand and age and type? They need to be identical in wear and everything else. The rears can be ad-hoc so long as they are sound and inflated correctly and in good condition.
If the struts or shocks were bad (especially up front) you would experience a feeling at specific (higher) speeds of loss of control, especially after hitting a slight bump. Sometimes a shudder will occur and last a few seconds as well.

Last, tires have a shelf-life. Even if you buy them “new”, if the tires you buy are not worn at all but old, they might be worse than used. The last group of digits on your tire code indicates the manufacturing date of the tire. So even if you bought this vehicle new, the tires may have been old when they went onto your car.

I would look at the u joint and differential. It will only get worse and more expensive so fix it now!

Try running your hand over the tread, especially the outer edge of each tyre. Does one or more have a wavy pattern that you can feel? I would also have a mechanic check to make sure everything is tight. A loose mount somewhere can do this kind of thing.

My best guess would be struts/shocks.