You might find that 31s catch with the shackle reverse as it pushes the wheel backwards towards the bulkhead during articulation. It's better to run shackles up front with larger tyres for a number of reasons. Personally I'd also try to avoid going spoa, there are a lot of problems associated with it on swb SJs, increased axle wrap, steep driveline angles etc, It really reduces the reliability of the SJ driveline in my experience.
I know it's just my opinion but in my experience SJs with less lift are a lot more reliable. I used to run 31s on an SJ with just 2" lift springs, lowered bump stops and some very minor bodywork tweaks. I have been running 33s for years on the current set up with 1" of lift and I have very few drive line issues, It's been a couple of years since I changed a UJ and I do 10000+ miles a year in my Sj, I know people that do maybe 10% of that mileage in SPOA sjs and seem to be always changing UJs.
I presume you have bump stops fitted? I used to run 235/75r15 treps on a completely stock SJ410, think all it had was some very minor front bumper trimming and it didn't catch anywhere during articulation