I noticed a huge difference in how smooth my car climbs from a cold start to 60 after changing the rear diff fluid at 117k miles. Diff was never touched previously. Before, on a cold start, going to 60 felt a little clunky. Now, 0 to 60 feels great. No more clunkiness. Before, I would often let off the gas a little because I hated feeling the clunkiness and the very slight shudders on the climb from 0 to 60. Now it's butter! Anyone else??

 

This is just me and I've spent a lot time around cars and IMO a standard diff doesn't do that unless something is wrong. You see metal flakes? Was it slurry thick or unusual in some way?
What did you put back in?

 

No metal flakes, just a lot of fine metal particulates attached to the magnetic drain plug.

 

Change my wife is250awd front and rear. To be honest not a difference but then again it was never a issue either. 173k first fluid change.