Well the Verdict is in. Inspection done.

Upper Rear "control arms" if that is what they are called, maybe they are a linkage?

Jacked vehicle up, rear wheels had totally visible play (I moved the wheel as he looked, it was shocking to say the least, like a shot wheel bearing it was that bad).
RR had Vertical play
LR had 8 o'clock to 2 o'clock play

Both caused by the same component bushing

Replacement part RRP $700ea (so he said)
This guy said he can get them for $170ea (so he said)

Checked front end, rock solid.
Checked tyre wear, suggested a few more pound air pressure in the fronts.

The part was not available on this page: BMW E53 Control Arm Replacements & Upgrades | Turner Motorsport
So i'm not sure where to look.
And i'd only look for interests sake, the business was super friendly and straight up, very honest people.
Be more than happy for them to use their parts.
Parts on the shelf in Melbourne, complete arm with both bushings fitted.

Then aga

in, if this is the parts, then i am getting raped a bit hard
BMW E53 Control Arm Replacements & Upgrades | Turner Motorsport

The part he pointed to, is more the top components in this listing though.
BMW X5 E53 REAR SUSPENSION WISHBONE UPPER LOWER ARM HEAVY DUTY LINKS BALL JOINT | eBay



Another link showing the part.
Rear Upper Axle Strut Control Arm Guide Rod Guiding Suspension Link for BMW E53 | eBay