The breakover angle on a stock XJ is about 21 degrees, which is about the same as the JK. The XJ should have about 9 inches of underbelly clearence with that breackover angle, the JK, a bit more 9.5 or 10 inches.
The suspension lift should add about 4 inches to the 9, the 33 inch tires about 2 more, so you should have about 15 inches of clearence at the belly.
A JK with a 4 inch lift and 33s would be slightly higher, you could also do a 3 inch lift and 35s. The 35s would give you better clearence at the axles.