Additional, possible bad news: you may need the redesigned piston if your ARB is a touch older (mine was, about 1998 vintage or so). It really is not all that difficult in a minitruck; it may be more of a pain in a clip-type axle though...
Tell tale is that the O-ring failure will show itself by losing pressure over a few minutes of engagement. Piston failure may not; it may just spew oil. You can always do a Cap't. Bligh imitation when it does....
"THAR SHE BLOOOOOOOOOOOOOOWWWWWWWSSSSS!!!!!!!"
Heb Dduw, heb ddim; Duw a digon
I'm that gun-totin', farm-raised, evangelical, pro-environment, OHV ridin'/drivin', Southern civil rights pro-labor Liberal yo' momma told you couldn't possibly exist.