It's really a very simple problem and solution.
Problem: you need to be able to deal damage in order to level or do things solo. Tank (and healer) roles require different stats, and thus different gear.
If your game is built around leveling up by yourself, people should not be expected to level/solo as a tank or a healer. These two things are incompatible and are the direct cause of shortages.
Solution: remove the necessity for different sets of gear. Players can specialize with things like runes or enchantments or whatever, or even with secondary stats, depending on the game, and thus still have to 'choose' a role to be best at and for their gear to be best-suited for, but they can still easily fall back to being able to do things on their own with the same gear in a different spec.
Blizzard finally figured this out with WoW recently, and has gear with base stats that switch depending on the current spec. Secondary stats are static, but being slightly worse at soloing isn't a major concern for basically anybody.
In games where you're expected to group together in order to level all the way up, this isn't necessary. But trying to have both just results in one or several aspects suffering. In the case of modern themepark MMOs, pre-cap at least, it's the group content that suffers.