I replaced mine at 100,000 +, I have 3 Valks all high mileage, there wasn't a dam thing wrong with any of the ones I replaced....I guess its just piece of mind, my thinking was, if I take it out to check it I might as well replace while I'm there..

...the next thing I'd keep a sharp eye on is the pinion cup and drive shaft spline going to the differential...pull the shaft out of the cup and make sure its being lubed....there's a seal on the end of the shaft, in all the times I've pulled the shaft out I have never had to replace the seal...
