Tom
The Drz has a standard 20mm cartridge. You can replace the entire compression assembly if you wish with a Race Tech unit. Its the same as for the XR400. You may also be able to adapt the original unit to accommodate a piston and shimstack combo. Earlier Drzs had a valve and shimstack, obviously they have reverted to a lesser unit. Thats progress for you. Contact Izzy from Suspensions are Us and ask him for an FAGV02. He should be able to hook you up.
Terry