I am a little biased as I run a gear shop - but I sell lots of gear sets to guys who think my install rate is outrageous and usually I hear "how hard can it be". Some of these folks are indeed succesful but a lot of the time the next phone call is "my differential is howling" or "your parts failed" etc
Most fail to recognize that we have been doing this successfully for 35+ years and pay our techs good money for good reason. I am also one of those "how hard can it be guys" but I also understand that it could cost me extra in the end and sometimes it does
I myself pay my techs to set up my differentials as I am just not qualified(partsman by trade not technician) just as I would pay somebody to build my engine.
Wow - I sure can ramble sometimes......................