I used a take off Nytro XTX skid. I'm pretty sure that either an Apex XTX or Nytro XTX skid will work the same, just different torsion springs and shocks.
It's been a few years since I did the skid swap. I know for sure that I had to change the rear arm mounting brackets to the XTX style brackets, and I'm pretty sure that I was able to use the stock mono skid front mounting plates but had to relocate them.
Overall it wasn't too difficult of a job. I really like the longer XTX skid in the Apex chassis.