Metallic oil isn't a good sign, however it could soldier on for a long time yet, as you've changed the VCU and it has matching tyres, which reduces the loading the IRD has to take.
The metal in the oil comes from the crown wheel thrust bearing, so there's obviously some where in that.
How long it last before the gears loose mesh is the uncertain part.
You could remove the propshaft as a temporary measure, which will make the IRD last much longer, but it means you'll only have FWD, and you'll also need to tell your insurance, as its a vehicle modification.
If you want a reconditioned unit, then you can currently use yours as a unit for remanufacture, but if the gears strip, and it goes bang, then unfortunately it's scrap value only, and you'll need to pay the surcharge on a reconditioned replacement.
Contact Bell Engineering, as they've got a good reputation, and have been reconditioning IRDs and VCU for almost 2 decades.