rear axle bushing
 

What do I need to replace on the rear axle to make it 'new'? I don't get any noise but I get the rear tires worn out. I bought the wishbones (part 11) but what else?

http://realoem.com/bmw/diagrams/q/e/230.png

You need to check the rear toe and camber and set it to within spec. You should also check those pesky rear subframe bushes to make sure the carrier is not floating about. Powerflex bushes for these are the way to go.

Powerflex

Can be found discounted on eBay.

You will almost always get heavy rear tyre wear though on the X5 due to the geometry but keep the pressures within spec and with the Powerflex bushes done and an alignment, I have now done 22000m on a set of tyres that are now about 3/4 worn.

part 6 and 11 in the diagram

As well as 2, 6 and 15, all of which control the rear alignment.

usually 6 and 11 go first causing the tire wear.