Love my LJ78. My comments are on the 1990+ models which are superior to the late 80's models in a number of ways.
The 1990+ axles are not the same as mini truck axles. The front axle housing is unique to the Prado, but the knuckles, axle shafts, brakes, birfields, hubs etc. are all the same as the heavy duty front axle. The rear axle housing is unique to the Prado. Yes, it has an 8" diff instead of the 9.5" in the heavy duty cruisers. But it is tough and reliable. I have never heard of them breaking under normal driving/4wding circumstances. Maybe if you couple it to a v8 with 37" tires and rev it out you might break them. The brake setup on the rear is the same as the 80 series. So basically the axles are NOT mini truck axles, but are a mix of cruiser parts and custom parts. Diff ratio in my Prado is 4.88 which is perfect for tire size upgrades.
Transfer case is a full heavy duty unit, same as in heavy duty landcruisers.
Frame is just as strong as heavy duty frames with just minor differences for the coil suspension and different engine/trans mounting. And BTW, 70 series frames are a huge improvement in strength and durability compared to the old 40 and 60 series frames! I often see this when out 4wding with our club. I've heard many a fan hit the shroud in the 40/60 series while the frame is flexing. Never in an 70 or 80 series tho. I have a tire in the air often, and my fan has never touched the shroud.
Transmissions are good. A343F auto is same as found in the late 80 series and early 100 series behind much bigger engines! R151F manual transmission is fine too, and is in fact used in later heavy duty models as well.
Engine is the only thing that is really an issue. Mine is proof that they can be improved and take a beating though. It just takes some money and effort to get them there. Not a whole lot more than you'd put out turboing a normally aspirated heavy duty model diesel motor though....
The coils are nice, and can stand up to a load fine as well if you upgrade them. I run OME suspension with heavy rear springs. I can load 1000lb in the truck, and have a trailer tongue weight of another 500lb and the truck sits level and drives just fine.
So they are fine trucks if you are mechanically capable upgrade the motor a bit. If you don't want to do this, stick with one of the heavy duty models.
Look for a 1990+ model if you can, as the 2LT-II is better than the 2LT, and I believe the axles/birfields are stronger too. Brakes are better as well if I remember correct. There are some other differences too that I can't recall right now...