Ride height!

Is it adjustable or with lowering springs?

It's not a million miles off with non-adjustable springs, within 5mm is good, 10mm is not untypical. Do you sit on the right or left in the car? Sitting on the right would bring it closer to level.

what spring rates are the springs?

It's probably typical then. The gearbox and transfer box are pretty heavy and on the left side of the car. This pushes your front left down a little which raises your back right up.

Up to you. It's not broken and it's not ridiculously low. With any springs its normal to have a difference and most coilovers I've seen aren't carefully setup to match ride heights/corner weights.

By all means change but I wouldn't base the decision on a 10mm ride height difference side to side