Thomas before deciding if it should be a 4×2, 2×1 or 4×4 you should define the term in your own needs as to what “Overlanding” is to you as it is a broadly used term but can vary between people as to how they imply it.
Overlanding can be tar and gravel based where you can use a normal vehicle with or without a trailer (vehicle design and personal requirements dependent).
Assuming that we all are referring to Overlanding as “remote country traveling on lesser used roads mainly” a 4×2 High rider bakkie/SUV with a lockable rear diff lock can do a fair amount of traveling of the more known roads into lesser traveled country.
If Overlanding is referred to as remote bush country exploring very remote and lesser places I will not consider a 4×2 as an option at all. Here I’m referring to traveling as Voetspore has done on certain parts of their travels.
The risk is to big to damage something on a vehicle, especially as the drive-train is going to be strained at certain points which might lead to problems/breakdowns in very uncomfortable places.
I have had a few 4×2 bakkies driving where I drive with my SFA, however the difference do come in traction force either for braking, steering and or maintaining momentum/grip to get through certain difficult sections. I always have full control over my vehicle and when traveling I know that when faced with a challenging section like a bridge washed away in Lesotho or an incline turned muddy on the uphill to a campsite, I can get there provided that it is not impassable or to risky to attempt in a vehicle.
The 4×2 has to work harder to get through and also steepish gradients do present problems for laden 4×2 vehicles when having to stop and pull away or even lower momentum in order not to bash the vehicle.
I have noticed this in Lesotho over December when I met n guy driving a Sani 4×2 2400 Petrol vehicle in Lesotho going to Sani Pass from Mokhotlong over the black mountain pass. He was turning up all the stones on a rocky incline on the pass and digging holes as he struggled to the top on the loose gravel and small rocks. I could see at one point he stopped and locked the rear diff as he lost traction and had to pull away and couldn’t get going before he locked it. I on the other hand was in 4H and was heavy loaded in the back with the tail town. I opted to select 3L 4×4 going up and 2L going down on the other side of the mountain and idle my way along following this guy digging holes and in general destroying the track that was already in such a bad state. I literally left my 4YEFI to idle up the mountain with no driver input at all on the throttle as the idle control valve did the work, but I could do it, because of low gearing and 4 wheels having traction.
Having done quite a few trips into different parts into Southern Africa I will say that you can drive to Lake Turkana in Northern Kenya with a 4×2 in the dry season, and will not have a problem, but what I’m trying to say is that as some point you might wish that you had either 4wheels traction or low gears or even both just to ease the passage of the difficult section.
I see 4wd traction something like a safety feature as Leon & Andy mentioned here. People on Overlanding trips tend to stay away from rough places that can lead to failures of vehicle components etc. However having 4wd makes it possible to venture further of the beaten track and when used correctly does not damage the track as much as a 4×2 pushing its way through a the same section.
Its all about what you need, what you can afford, and also how far do you wanna really go into the unknown. A 4×2 driven correctly can get you further than expected but it will not be able to hold its own if the conditions unexpectedly turn around and become washed away sections or even slippery roads.
I have seen many 4×2 vehicles getting “stuck” on slippery ground where the wheels just could not provide any more traction even with mud terrain tyres fitted, just clogged up where I just locked up my SFA in 4×4 and drove the same section with partly worn All terrain tyres also clogged up with relative ease even with a load in the back.
Also fuel consumption is negligible between a laden 4×2 and 4×4 of same size/load/ engine : ie Hilux 2.7i 4×2 D/C , 2.7i 4×4 D/C. The 4×4 might be about 5% heavier on juice on a overall trip but it will do it with so much more confidence and reassurance that even if parts are difficult or traction is compromised, you should be able to get through it a lot easier and with less vehicle and track damage than the 4×2.
If you want to drive with peace of mind, buy the 4×4 version and use it correctly. 4×2′s are great campers and towing vehicles for caravans etc, the people that go to the national parks etc and do not need the extra traction as they drive graded dirt roads or tar roads.