That isn't true at all. You don't need to remove the ring gear, nor the pinion gear. Why would you even think of touching the pinion gear?????
All that is needed is to remove brakes, ABS sensors, diff cover, c-clips & the axles, then remove the carrier, then remove the S-clip, take out the spyder gears & install the new posi unit.
It really depends on how hard it is to remove the spyder gears & install the new posi. If it's easy, then my guess is around $250 for 4 hours of labor. New lockers are really easy to install. But I can't comment on a new posi.
Much easier than installing a new ring & pinion IF the new posi unit isn't too difficult or needs special tools.
Why wouldn't you want to do this yourself? It isn't like you need to mess with the backlash/pinion depth of the R&P gears. My guess is a full set of sockets/wrenches would do the trick. Just see what is all involved with installing the new posi, and you will have a good idea of time estimated time involved.
Scott