Well I have a leak on my cruisers front 3rd member after some mild wheeling this weekend

its about 1 drip per hour, I checked my birfield grease and its still grease and my breather line seems to be clear. I took it into the Independent but authorized Toyota service center where they did my axle 15,000 miles ago and they found that 5 of the lower bolts on the 3rd member were slightly out of torque and that the grease is leaking from the lowest bolt. My guess is that they didn't torque the 3rd member to spec and when I locked the front diff it torqued the 3rd member in the housing, tearing the paper gasket. So, question: what is the mileage or time limit I can reasonably expect for guaranteed work from a shop like this? Its been less than a year and, as i said, about 15,000 miles. replacing the gasket is a BIG job.