Tranny fluid at 30K is a good idea IMO.
I changed the diff fluid early on myself, just to get the factory fluid out of the diff along with any shavings that may have accumulated as a result. Otherwise, with the synthetic gear oil in place you should be good for a substantial amount of miles depending on what you tow.
The spark plugs are kind of subjective to me. I know the manual says 90 - 100K for the spark plugs, but I have always felt iffy about leaving them that long. I had the same interval specified on my Chevy for spark plugs, and I ended up changing them at 60K just to be on the safe side. Bottom line, go by what the service manual dictates NOT what the dealer service schedule says (they'll have you in there every 10,000 miles for something if you listen to them).