Fer whatever it's worth..... third is the first to go (taking 3rd and the cluster). The bearing on the input shaft is next to go (this takes out the input gear and cluster, if you do not catch it early). And, 5th, is usually next to go, taking out (sometimes) both gears and sometimes the housing. Most folks say it's because of weak parts. But,... there are pleanty of conservative drivers, with stock engines, that easily make it over 200K miles. I have one (female) customer that has 246K miles on an 85 cj, and she has never had 3rd fail. She once had the front (input) bearing give up, and it was caught early enough that she still has all of the original gears. She drives fast (my opinion), but she is obviously not very hard on the gears. She has had one clutch replacement, and one engine failure (water in the oil).
bobH