Regarding Aunkai, be careful with the exercises. There are details not explained in the DVDs. Fortunately there are a couple guys in Seattle who hosted Ark in the past and explained some of them to me. If Tenshijin is killing your back I'd be concerned you're doing something wrong - i.e. not stopping at a body limit and letting you're structure stretch and slowly adapt. but I could be mistaken.
I actually pointed this out on aikiweb after watching some aunkai excercises a few years ago. I was surprised no one else did at the time. People are far too casual about these excercises IMO.