What it says on the tire is the pressure for maximum load, which you don't have anywhere close to on an XJ. If you're running 30 on a set of bias ply 35's you're probably running too much.
The corrent tire pressure is whatever it takes to have a full tread foot print on the ground. If you don't see evidence of the tire hitting the ground all the way to the edge of the tread, then you have too much air. Use just enough to get full tread contact on the ground, otherwise you'll either wear out the edges from too low or wera out the middle from too much.
For 31's, 30 lbs might be OK. Street presure on my 37's is 22 lbs.