The 181 Front Beam-design Iron (FBI) axle consists
of a cast iron differential housing with axle
shaft tubes extending from either side. The tubes are
pressed into the differential housing and welded.
The integral type housing, hypoid gear design has
the centerline of the pinion set above the centerline
of the ring gear.
The D30 is both the 181FBI and the 186FBI. However, the 181 is a high pinion, reverse rotation where the 186 is low pinion, standard rotation. I believe the 181 was used on XJ's until the 2000 model year and is more desireable.