@MidMan I'll try to explain the Viscous Coupler (VC) topic a bit even though I'm less qualified than many to do so.
he viscous coupler, if your 80 has one, is in the transfer case and was a core component of the All Wheel Drive functionality for the 80 series land cruisers equipped with them. In the North American market most (if not all) of the FZJ80s were equipped with the Viscous coupler from the factory (after 92 perhaps, but I don't remember exactly). The "all wheel drive" 80s with a VC don't have the ability to be shifted into two wheel drive and don't have front locking hubs, etc.. Globally there was a lot more variety and some 80s have part time or 2wd/4wd operating modes etc. (as I understand it) and some 80s didn't have the viscous coupler from the factory even if they are full time 4wd (on some level). At this point many of the original Viscous Couplers have stiffened or locked up and then been removed so you have to factor in the original setup as well as any changes possibly made to ascertain if you have a VC in your truck.. etc.
The VC detects differences in speed between the front propeller/drive shaft and the rear propeller/drive shaft. In other words if the front wheels are spinning while the rear wheels have traction or vice versa then the VC detects this and locks/stiffens up to balance the delivery of power between the front and rear axles.
If you have a VC and have a taller/shorter tire front or rear then this could lead to the VC engaging to some degree all/most of the time. This will probably cause the VC to overheat and fail in short order.
I am not sure that a small fraction of an inch on one tire will matter but I know that an inch or so or more on one tire will matter. My 80 had a VC and mismatched tires when I bought it and the VC was locked solid due to overheating which I'm pretty sure was caused by the mismatched tires on the truck.
At the end of the day you need to keep your tire sizes as close as you can to avoid potentially burning up the VC in your truck if you have one and to avoid overworking/damaging your other differentials. New tires are probably cheaper than the possible towing, drivetrain parts and the repair labor in the long run.
If you try to figure out if your truck has a VC I think that the transfer cases that originally came with VCs are actually larger in that area to make room for them so that could be a clue among the many others. Testing by lifting some/all of your tires and spinning/etc. can also help you figure out what you have and how it's working. Beyond that researching your model, etc. should probably help you figure out what you should have at least.
