If the IRD had seized/failed then the vehicle would not move.
Check the condition of the VCU support bearings and the rear diff rubber mounts.
To check if VCU is seized:
car in 2nd gear
handbrake off
jack up one side so 1 rear wheel off ground
put socket and long breaker bar on that rear wheel centre nut
lean on breaker bar
If it moves slowly and smoothly with a fair bit of pressure then VCU is working.
If it wont move at all, then ducked.
If so remove VCU and prop before you duck the IRD and run in front wheel drive only. This will not damage anything!