I had to sit and listen to that sales pitch when I bought my 4Runner. They wanted $895 for all services up to and including 55,000 miles over 4 years. I won't put on 55K miles in 4 years (30K at most), so it was silly to pre-pay that far out.
I found the brochure I have with the fine print. It says that all regular maintenance items are covered according to the maintenance schedule. However, it does not include any service that's listed under the section "Additional Maintenance Items for Special Operating Conditions."
I notice that at 30K, the actual changing of the diff oils and transfer case oils (not just inspect, but change) are listed as these "Additional Maintenance Items".
It sounds like buyer beware. I've said this before - never just ask a dealer for the "30K service" (or any other interval). Always find out what they really plan to do for the price you're quoted. In this case, it sounds like maybe they "normally" change the diffs and transfer case as part of their 30K service - that's how they're hitting you for extra money. Or, maybe they just do that to people with coupons to get some actual cash out of the deal.