Disco's from 1996 to present do not have Distributors, they are distributorless, and are basically tuned from the ECU, like most modern cars they do not have a Distributor any longer.
tune up for you consists of plugs/wires/air filter, oil, oil filter, and grease anything thing that can be greased. Never hurts to change transimission fluids/transfer case fluids and Differential fluids on a regular basis. Every 30,000 miles or so.
Are you having issues or just wanting a tune up? Plugs = Champion Platinum on the D2's.