Your rear castor is not a problem, because there really isn't such a thing as castor on a rear axle. Castor refers to the angle of the steering axis, when your rear axle is not steerable, therefore, castor angle is meaningless.
What you're probably worried about is the pinion angle, which can cause driveline vibration. The other reason for replacement rear trailing arms is for very long travel systems, where the arms can bind by crashing into the frame, therefore bent arms are used for clearance.
People do sometimes replace their front arms as well. This is to correct the castor angle, and the pinion angle would follow. Probably not many people do this because it's expensive, and not really required for moderate lifts (~2").
Now, one question is, why did you buy spring retainers? Unless you have long travel shocks (which you don't appear to) then you don't need them.
Secondly, why did you buy the spring isolators? See, the spring retainers completely eliminate any benefit from the isolators. The isolators are supposed to eliminate metal-metal NVH pathways between the axle and the frame. So you put rubber between the spring and axle, but then the retainer bridge that gap with more metal.
I have a 2" OME lift on my D2,, along with the HD shocks. I haven't corrected the castor or anything like that. Steering is slightly less precise, but I haven't noticed any effects worth worrying about. I do know that when I put my motorcycle on the hitch rack, which end up dropping the rear and lifting the front a bit due to the tongue weight, that the steering does start to enter the "spooky zone", but not so much that I can't live with it.
As for the rear... well... that's what HD suspension is going to give you. No way around that. Even fully loaded, those stiffer shocks will create a stiffer ride. Lastly, the spring rates are not different from one side to the other, rather, the spring load rating differs, but not nearly as much as has been reported. I measured the difference at closer to 1/4" as opposed to 1".
