I was changing out my driveshaft rear flex disc recently and I noticed a small "seal" like device inside the rear of the driveshaft tube. You could see a tiny annular spring inbedded in this seal. As luck would have it, this seal had dried up and had a chunk of rubber missing or torn away. I might have gouged it on the axle tube during removal. Is this part critical in the functioning of the driveshaft? What purpose does it serve? Talk about over-engineered. I'm guessing it's just a dust seal (?)

I put everything back together already, so I hope this thing won't come back to haunt me later. I couldn't seem to find this part on any of the major parts stores online.

Thank you again for any help here.