It means Intermediate Reduction Drive. It's the component in the 4X4 system that takes the drive from the gearbox, and directs the drive to the wheels.
Unfortunately, yours is a familiar story, which repeats itself over and over again. A Freelander can be reliable in the 4X4 department, but it does need any previous owners to have actually followed the handbook's instructions on tyre maintenance. The only reason for premature 4X4 system failure is due to tyre maintenance incompetence.
This tyre maintenance is crucial, or serious 4X4 system failure will occur.
Now there are several components that can make the noise you described, when they fail. Some are easy and relatively cheap to replace. However some failed components are extremely expensive to replace, which often renders an older or lower spec Freelander as beyond economic repair.
It is often possible to simply remove the propshafts and continue using the vehicle, however if the rear diff has failed, this isn't advisable, as it can seize up, causing the rear wheels lock solid, which is extremely dangerous.
So the who system needs checking, preferably by someone who knows how the Freelander's 4X4 system works, and how to test it correctly.