Over time the seals will start to leak grease and if the moly grease in the knuckle gets too low, the birfield joints will wear excessively. My neglected 80 only had about 85,000 miles when I bought it, but because the previous owners let the grease get too low, both left and right birfields were clicking on sharp turns. One common cause for leaking knuckles actually starts with a clogged differential breather. When the axle and gear oil warms up from the friction of normal driving, they expand and the warm air can escape through the diff breather. When that is clogged though, the next path of least resistance are the oil seals on each axle. When gear oil gets pushed past the seals, oil mixes with moly grease in the knuckles, making it thinner and more likely to leak. Also when the axle and the air inside it cool and contract after parking, air can’t enter through the breather, so the mix of gear oil and moly grease gets sucked back through the oil seal, into the axle and differential. If you change the gear oil and it looks like army green brownie batter, this is probably the cause.
How to Rebuild Knuckle on 4wd Pickup/Hilux, 40/55/60 Series Land Cruiser:
How to Rebuild Knuckle + Replace Birfields on 80 Series Land Cruiser: