The 3905 diffs are "supposed" to be better than previous diffs. I'm not sure exactly what that means though. Maybe trx specified some tighter dimensional control so there is less possible slop between the way they go together. There's a lot to be said for "if it ain't broke, don't fix it" too. Run it til something starts making noise, then worry about it. It could be a long time.

I bought a pair and didn't open them, just installed them. When I get caught up, I'll think about pulling one and taking a look.

The more expensive diffs like UE - shimming is critical to getting the most out of your investment. I wouldn't even think about not shimming a set of those.