As far as the differentials go, I would personally use synthetic gear oil in them. I have seen far too many differential failures at the dealership from using conventional oils. Especially the front diff. The transmission should be power flushed. Dropping the trans pan and changing the fluid and filter only gets about 1/3 of the old stuff out. The transfer case may need a special fluid from the dealer if it is auto 4X4. Synthetic is not necessary in the trans or transfer case. You will spend a little more for the gear oil, but it is well worth it. Less wear and tear on the bearings, as those are the weak parts. Just my opinion and advise. I worked at a GMC dealership from 2003 to 2005 and saw alot of the problems in those years.