This happens to all sorts of motors! If you google "coolant in gearbox" (or similar) you will see. I had it happen to my 90 recently, but fortunately I didn't suffer any transmission damage (I caught it early). What happened is an aluminium autobox cooler union (inside the main rad) corroded and came loose. This happened despite me believing that I had quality long life coolant (pink stuff) which is supposed to prevent the aluminium corroding (which happens much quicker in water). This allowed the coolant and atf to cross contaminate. Not ideal for the cooling system, but apparently really bad for the autobox (the water based coolant dissolves the auto clutch bonding agent - I am not sure how quickly this happens though). The transmission specialists will ALL tell you that the gearbox is toast if this happens, but I flushed both my gearbox and coolant system, and my truck has never run better. A word of warning: I almost paid £200+ to have my autobox "flushed", but fortunately realised (just as I was handing the keys over) that all they were planning to do was drop 4-5L of atf and top up again. In the UK very few transmission places have the special pump to properly flush the box. You can however do it yourself if you have a bit of patience, by using the transmission pump to pump out a few litres at a time and topping up (repeatedly) - this is what I did and used about 30-35 litres of ATF before it ran clear.
I'm not sure about the 100 series, but on the 90 series I would suggest changing the rad at about 120-130k miles (mine went at just under 150k) and only using good quality coolant.
