The IRD is 'driven' by a slpined input shaft from the gearbox. Directly driven from that shaft is the PTO for the propshaft to the rear axle. As the PTO is turning, we know drive is getting into the IRD.
At the end of that input shaft is a differential driving the front axle. Its a 'standard' open diff - so it will send torque to the wheel with least resistance. So somewhere between the diff and 1 of the wheels is broken - meaning there is no resistance and the other wheel will not be driven.
The driveshaft for the RH wheel plugs straight into the output from the diff. This is known to fail due to wear on the inner CV on the driveshaft. The CV wear causes wear on the output support bearing in the IRD and drive can be lost.
The driveshaft for the LH wheel plugs into the gearbox casing. The IRD output is picked up by a short splined shaft that goes through the gearbox to the driveshaft. The splines on this shaft (or output from IRD) are what
@Nodge68 is referring to.
Pretty sure the IRD will need to be removed to find the fault - definitely to fix it.
Oh and it's a Land Rover, not a JEEP!!