Every class can heal itself one way or another.
Every class can mitigate damage one way or another.
Some are better at each of those things just like some classes are better at dishing out raw, sustained DPS over a long period, but every class can do that. Every class can solo (and I mean truly solo, not just solo-with-hireling) a tremendous amount of content in DDO.
Rangers are considered gimpy because many of the things that they can do on their own do not adequately complement the abilities of other party members in a way that is particularly useful in most quests and raids.