Difference in suspension height
 

I noticed recently my right rear suspension is higher than the left side.

The difference is pretty visible to the eyes. The right is about 4.5 fingers while the left is about 3 fingers.

It is a 2011 XJL with about 55,000km miles on it.

Do you guys think it is normal or any way to reset it?

The rear axle of the X351 has air springs with traditional coil springs at the front (unlike the X350 which had air springs all round). The height is monitored by a sensor at each wheel & the rear axle height is controlled by the Air Suspension Module. The left & right side are adjusted individually based on vehicle load.

You will get some variation in the rear ride height (factory specs are +/- 12mm, slightly less than the X350) the usual reasons include;

Vehicle not on level surface
Weight added/removed from the vehicle without the engine running
Height sensors dislodged/moved

Or if the X351 is anything like the X350, the calibration drifts over time...

The only way to reset the ride height is with the dealer-level diagnostic system, known as SDD. It's a laptop with proprietary software, and a suitable interface cable.

A dealer would be able to sort out the ride height for you, or an independent workshop that has the equipment. Some forum members also have this equipment.

You might also be able to fudge it by manually adjusting the height sensors, either by adding shims under them (lowers the axle) or by bending the mounting bracket (i wouldn't recommend this, but others have done it).