Whats the car weigh,how fast is it? 60 foot?.....4 years is pretty good if you've gotten 200 plus passes.I also run a solid pinion spacer on all the rears I've built.Hard launches can sometimes cause the crush sleeve to crush alittle on it own and then the pinion nuts loose.Some people then try and tighten it only cause more damage as the pinion bearing preload has changed. Also pro gears will wear and break and usually last about 200 passes. I run 8620 gears.their cheaper,and last longer for my combo.