35's on a D30 will push it's strength limits, but many folks run them and carry extra 760 jointed shafts, upgrade to Warn hubs w/alloy shafts, etc. This gets really expensive, and most feel that it's cheaper to just build a D44 for the front than sink extra money into a D30 to run big rubber.
That being said, one can run 33's or the metric equivelent (I prefer 285/75/16's) and driven sanely, the D30 will hold together just fine.
What year is your rig? For an SYE cost reference and additional Tech, check this site out. Tom is a local, as well as a U4 and NAXJA supporter and he knows his stuff.
http://4xshaft.com/ Also note the Dec 10% off deal.
Also, drop by NAXJA for more XJ Tech, plenty of locals running XJ's that have been there, done that to provide feedback, lots of folks in the know here on RME as well.
We have a shop day coming up on Jan 22nd where folks with various XJ builds get together and help each other wrench on their rigs, installing everything from a new CB antenna to complete lift kits and just about everything in between.
Check us out
http://www.naxja.org/forum/forumdisplay.php?f=72