Vampires (and perhaps Ghouls) have the best hook into Druidism, since, unlike many other undead (particularly Liches), the vampire ends up still tied into natural rhythms, such as the need to feed.
It's not like one can ban someone from being a druid for becoming something 'unnatural,' since, by definition, it's not possible to become something 'unnatural.' If it wasn't natural, it wouldn't exist. And since, in the AD&D universe, undead empowered by negative energy exist, they can't be any more unnatural' than living beings empowered by positive energy (yet another non-aligned energy from another dimension).
All life in this setting is 'unnatural,' in that it is empowered by energy from another dimension, and occupied by spirits that leave the 'natural world' entirely to go to other dimensions upon death.
If the druid continues to serve and revere the forces of the natural world, it shouldn't matter if it's animal, vegetable, mineral or undead. It's all 'natural,' in a world where creatures are either empowered by positive or negative energy.