[ctw41]

The Ring and Pinion along with the spider gears is in the center part of your rear axle along with supporting bearings. You have the axles going out from there supported by bearings with seals at the ends. The "third member" is usually referred to as the center part I mentioned. What goes bad are the bearings and usually teeth get stripped off the spider gears, ring gear or pinion gear or all of the above.
I would get a second opinion to be sure. My 2000 tundra 4wd has 114000 miles and still holding up fine. In most cases as long as the seals stay intact and your fluid is the right level and clean you can go hundreds of thousands aof miles without a problem