The only periodic maintenance the factory service manual recommends at 20,000 miles is repacking the steering bearings (actually recommends it at 16K miles) and changing the transfer gear oil.
It also recommends changing the sparks plugs every year/8K miles, checking the valve clearance every 16K miles, and replacing the brake hoses every 4 years. I personally think all of those are unnecessary to do so often, but that's what the manual recommends.