Dinks, apols for taking the **** the other night about your "Mondo mode" question. T'was the ale talking! I can see you've possibly got some serious concerns so I shall attempt to give you a straight answer to the best of my ability
If you've not got a rear prop on there at the mo (so just running in 2WD mode) then visually, I *think* you can take the pinion housing off the back of the IRD and have a look at the crownwheel, etc, inside. It may not reveal much, but you possibly might see a chipped tooth on the gear. Don't think you'll be able to see much else though. Someone else may know better?
If it IS currently in 2WD mode and all is driving well like that then, as Freelaner sez, that bit's obviously OK. To verify the rest of its operation you'd need to connect a prop and VCU on there (a known good VCU...)
You could change the oil on the IRD and see if it comes out clean or full of bits of metal, which would indicate a problem. Make sure you undo the filler/level plug first tho (on the back of the IRD, about halfway up) before draining the oil as the filler/level plug can be a bit tough to remove. It sort of welds itself in place.
As I understand it there's not all that much in the IRD - the crownwheel and pinion gears (?), some shafts and about 8 bearings and a few oil seals. Sadly, the gears seem to shred and the bearings collapse quite easily if the drivetrain is abused (knackered VCU, more tread on the front tyres than the back, etc). I believe that IRDs were made a bit stronger post 2001 though. The oil seals can wear out as well and cause leaks. The most obvious is where the O/S front driveshaft goes into the IRD.
Hope that helps.