[Rory McNeil]

I think the problem is just the nature of the beast when used with such low ratios. I run a Dana 60 in my 3100 lb big block, 4 speed Jerico bracket car. When my buddy converted his 66 Chevelle from a bracket car to a SS/F stick car, he gave me his used 4.88 Richmond "Pro" gears, which after 10 years in my car still look mint, launching at 6000-6600 RPM best 60 foots of 1.29.
My buddy used the Dana in his Chevelle as a Super Stocker for several years, normally using 5.86 Pro gears, although he did use 6.14s at alttitude tracks. In SS/F trim, his car weighed around 3550 lbs, and ring and pinion breakage was fairly common, normally starting with cracks in the pinion. After new 5.86s were no longer available, he started stockpiling used sets, but eventually replaced the Dana with a 9", simply due to ratio availability. I think between the 5.86 and my 4.88s, there are 2 less pinion teeth, so I`m guessing that is the main concern with gear breakage.
Same thing with my Mustang, with a stick and 8.8 rearend, 4.88s last "forever", but I have broken a few sets of 5.13s.