FWIW, in my 30 some years of working on vehicles and others its served me best and kept me out of the auto parts store some, to not fix what ain't broke.....

I too used to be all about preventative maintenance or PM, but have found that replacing too many things at one time when not necessarily needed can induce problems...my .2 cents

I re-did the gasket on the rear diff. I did a half-a**ed job because I was pissed and the poor workmanship ensured that it had a leak. This time, I gently sanded the mating surfaces, made a proper bead of gasket material and did it correctly. This won't leak now.

The front diff took +2 qts to change. Buy 3 qts and you will have enough.

The transfer case took almost exactly 2 quarts of the "GM Performance Auto Trak II Transfer Case Fluid". This stuff is very thin and blue. It appears that in the past, someone put straight gear oil in this transfer case. Either that or the blue burned to black.

Today, I plan to pull the tranny pan, replace the tranny filter and refill with new ATF. I will report back how much it took to refill. A Google search turned up quite a few different numbers.

The tranny took about 6 qts when pulling the pan, replacing the filter and then refilling.