Two things:
First, why'd you ask this of Mark Hinkley? Your question suggests that he thinks a D30
would hold up to the abuse that 38's would certainly put to one. I can't recall Hinkley ever owning a set of 38's much less on an XJ. Hinkley would probably be a strong advocate AGAINST going bigger than 33's and he's got the list of "gettin' it crew" trails to show for it - his XJ's are usually among the least lifted, smaller tire'd rigs on those runs - and he has very little difficulty going where the big tire'd, tube buggies go.
Second, you haven't told us what kind of wheeling you plan on doing with your rig. A rock rig is certainly set up different than a mud rig and so on.
I have 35's on a "super-turd" D30 (297 shafts, welded tubes/pumpkin, trussed, Detroit locked, Phat Jeeps Canyon Climber steering, and a '99 Durango steering box with external cooler) with a D44 rear axle.
I would never consider going bigger than 35's because the sheer weight of bigger tires when in motion would so easily break the pinion, u-joints or locker just because of the small size of these parts in the little D30. I run 4:56 gears and could use to step up to 4:88's but I'm cautious because the pinion is quite a bit smaller than what I've got now.
I'm also at about 8" lift with the R.E. drop brackets. Going bigger than 35's seems to me that a guy would pretty much have to go long arm in order to get more lift and articulation to make the bigger tires be an advantage. Additionally, the bigger the tire, the more your turning radius is reduced. The D44 front may help recover some of that radius - not sure, but the D30 would be hurting.
Tell us what kind of wheeling you wanna do - that'll help us target your question a little better.
Also, If you haven't already, check out
www.xjdb.com and type in 35, 37, 38 in the search field and it'll pull up pics of rigs with those tire sizes - you'll be able to see what others are running and the kind of wheeling they're doing.
Hope this helps!