I've got a 1991 XT6 with the Air Suspension. Any chance there is a conversion to a regular strut??

Yes. There's a thread or two here if you search for it. Best bet is to go to the discussion board at xt6.net. There are some sticky threads at the top called Huck's ultra-secret suspension idea, both front and rear. They tell you how and have lots of pictures.

 

Basically, any late-80s rears will work. Lots of fronts work with a little modification. I have '01 Impreza 2.5RS fronts on mine.

yes you can do it, i don't have time to post but check out http://www.xt6.net