Breaking a birfield is far different from having one wear out. I agree you will probably never break a birfield unless you are wheeling hard with big tires, I wheeled the snot out of my '97 with 35" tires and never broke one. However, you can wear them out, which is a totally different problem. The birfield joint is just a CV joint, like any Honda/Toyota passenger car has. The difference though is on a passenger car the CV joint is surrounded by a sealed boot. This boot is packed with grease and since it's sealed the grease can not escape, so the CV joint always has grease pushing it's way around it, a very good thing for a CV joint. Because of the grease always being there normal life for a passenger-car CV joint is typically 200K+ miles with no maintenance at all, since there is nothing you can do to it. However, if the CV boot rips, then the joint will start flinging grease around and very quickly will have no grease and will start clicking, wearout, and possibly break, even if it doesn't break, once a joint is clicking the damage is done, the CV joint is unfixable, so packing more grease in there and putting a new boot on will not fix the clicking and the real problem, the real problem was it ran out of grease and the joint had to work without grease therefore causing wear.
On Land Cruisers the CV joint is not enclosed in a sealed rubber boot, I personally think it should have been, but it isn't. This means that you can pack the CV joint as full as you want of grease, and pack the surrounding cavity full of grease, and in 60-80K miles, you still need to go back in and cleanout all the old grease and put new in. The reason you need to do this is when grease is used (smashed into the nooks and crannies of the CV joint) it will start to break down, when it breaks down it basically means the oil is draining out of grease (grease is mostly just high-grade gearoil with a thickener in it). So when the grease starts to break down, it gets very liquidly, that is not a problem with a CV boot that is sealed (where can the liquid oil go? nowhere), so even though some of the grease turns to oil a bit in the boot, it's all still IN the boot, so still getting squished through the joint, providing alot of protection. In Land Cruisers the knuckle is not sealed, therefore when any grease starts to breakdown the liquid can drain out of the knuckle. This is bad because the more that happens, the less grease is left in the knuckle. The knuckle on Land Cruisers has felt wipers which help keep the grease into the knuckle, however, they are not designed to keep in gearoil, and they will not, which is what causes seeping knuckles (dripping goo from the steering knuckle).
Another cause of seeping knuckles is if the inner axle seal starts to leak (this is the more common way seeping knuckles happen in my experiences). The inner axle seal is a seal that rides on the axle and keeps the gearlube on one side (the diff/pumpkin side) and the grease on the other side (knuckle side). If this seal starts to leak, then it allows gearoil from the diff to drain into the knuckle, this will wash out the grease in the knuckle. This can happen alot easier than you might think, on my '97 I did the knuckle rebuild at 65K miles, and the passenger birfield was perfect, plenty of grease, inner axle seal was fine (still replaced ofcourse, you do not want to pull it apart again to replace a $4 seal), so I was kinda bummed about doing the work. Then I got to the driver's side it was trashed, there was not one speck of grease in that knuckle, the gearoil had washed it completely clean, so it was absolutely necessary to rebuild the knuckle, re-do the inner axle seal and back the birfield with new grease.
Interesting note is I never saw the driver's side knuckle leak anymore than the passenger, yet apparently it did, and this was on a low mileage vehicle with only 65K miles.
On your '94, which would be considered a high-mileage vehicle this is even more important. There are very few trouble-spots on a 80-series Land Cruiser, but one of them is the front axle and the birfields, they are not a bad design, but they do need maintenance. Some items really never wearout, wheel bearings for example, you clean repack every 100K miles and I'm sure it'll be happy forever, but a birfield is not like that, they will wear out, but you can prolong their life by doing the knuckle rebuild and birfield repack at normal intervals. New birfields can ofcourse be purchased as well as anything else in the frontend, but it will not be cheap. If you're dieing to spend more money on the vehicle, then by all means don't do the birfield/knuckle work, wait till it gets really bad, then shell out the money for all the parts then. However, you can't say "well this Land Cruiser finally broke", because it didn't break, the owner of the vehicle is who broke it by not doing the scheduled maintenance to keep it running.
Just my $0.00 worth...
