Yes, ARB's are in the future plans along with HD axles, Bill at GBR is checking into the availability of HD axles for the P38. I am leary of running the stock axles with an ARB and 33" tires, but I have heard that the current Defenders use the P38 rear diffs and I believe they handle 33" tires with no problems. But better safe than sorry.
Might also want to find a 4.6 if you can. The 4.6 will have a much better response if you go to 33"+ tires. My poor 4.0 with 33's and stock gears is just barely adequate, but really doesn't bother me. 4:11 gears for the P38 are now available from GBR.