Installing airbags to counter the lowering effect of shackles seems a little counter-productive to me. I would junk the shackles, level the trailer using a different position on the hitch, the way it is supposed to be done, then readjust the WD for the correct front fender height and forget about the rear.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner