And before you ask, yes you can convert a pickup to 6WD if you really want to go through the headache. The easiest thing is to add a dummy axle in the back that is not driven. A lot of medium and heavy trucks have axles that are off the ground and are either lowered in place by air or forced down by the load to help support the weight. A truck with a dummy axle on the ground all the time can find itself in situations where there is not enough weight on the driving axle to keep it from spinning.
A driving axle is much harder since very few light truck axles have a "through drive" to allow and axle behind it to receive power. I have seen a few 9" Fords fabbed up to use a pinion in the rear and a couple other axles but I think all were custom projects. The other options are swapping to Rockwell axles from a M44 series 2.5 ton military truck. The input and output on these axles is on the top, so they require a lot of lift to clear the chassis and especially the engine in the front. But since the axles are all the same configuration, you can drive any of them from the front or back. The M135 series military trucks do also have driven tandems but use a more conventional design axle with the yokes in the front of the rear axles and have a pillow block on the forward rear and a shaft to transfer power to the rearmost axle. Th transfer case has dual rear outputs for this setup so you are forced into using this very large and heavy transfer case with this setup. Another note on the M135, the front axle is backward, meaning the drive shaft rotates backward from all common front axles. So again, you are pretty much forced to use the transfer case with that setup.
Another setup is to use a second transfer case or transfer box to transfer power to the rearmost axle but you have to run the transfer case at near vertical and this can cause a lot of lubrication problems that will need to be worked around. A transfer box is pretty much a custom deal and very expensive. The now (rightfully) out-of-business USA6X6 used transfer cases and a custom transfer box as well as the Rockwells with mixed results. Unless they farmed out the work for their transfer box to a quality shop, I would not trust it.
Now for some points on 6WD. The main problem with 6WD on a smaller vehicle is ground pressure. When you add more points of contact to the ground that support the weight of the vehicle, you spread the load over all those points. So a truck that weighs 6000 lbs and has 1500 lbs supported by each tire on level ground would only have 1000 lbs on each tire in a 6x6 configuration. Naturally in this simple example I am not taking the weight balance of a real truck into consideration. In most vehicles you would actually end up with more weight on the front and less on the rear axles. Now you take that truck and put it in some off road obstacles and transfer the weight around and you can easily end up in situations where you can't climb because there is not enough weight on the tires to get traction. In a lot of cases a 4x4 truck would preform much better than the same truck converted to 6x6. Now if you are adding a lot of weight to the truck and are exceeding the load rating of the two axles under the truck, you could convert to three axles to support the load and still have decent ground pressure. This is going to be sort of rare in a light truck since the frames will only support so much and you begin exceeding the design parameters of the original vehicle. To say the least, a 6x6 conversion requires quite a bit of math and thinking to decide if it's necessary or is going to function as well or better than a 4WD. Most are done for show or they quickly find that their truck is only good in light mud or sand where low ground pressure is advantageous.
There are other points to consider with 6WD conversions such as tire scrub. The rear axles do not turn so you will slide or drag most of the rear tires in any tight turn. This can cause a loss of traction on slick surfaces and will easily decrease the life of the tires. Another point is when you do have a working 6WD vehicle that does get good traction, you can overpower the front axle and push it out of a turn in certain situations. Your turning radius will easily suffer and in some situations you will not be able to turn at all.
There are other points to consider and think about, but that is the core of the matter. Now, does anyone want to cover 8x8 conversions with dual steering axles? I've thought my way all over that one also, lol.