Paj Gen 4 LWB rear Diff Bushes

[Tony M]

I was inspecting the bushes on my Pajero 2 days back when I noticed the rear diff bushes were not identical.
Is this normal?
I purchased the vehicle second hand and don't have a full service history so I'm not sure if they were previously replaced.
(Still struggling with identifying a rhythmic 'wumm, wumm' noise above 100 KM/Hr under gentle acceleration)
TIA!