I have a 99 Discovery II and I'm having problems with my rear right suspension, I think the air bag is leaking because I can hear it when it daflates, I have to wait for the left side to deflate, so they both get to the same level and then yes I am able to use the button in the dash to raisethe rear. It works and then in a few days again the same problem.
I'm thinking of changing the rear bag, I found it on line for $ 141.90, new.
Am I right making this change?

 

Well with the airbag full take soapy water and spray around the airbag and look for the leak. It will bubble around the leak. It could be as simple as a loose fitting on the air line going to the air bag.
http://www.roverparts.com/Parts/9268D2ABP.cfm
With this you can fix it forever.

 

If you're intent on keeping the bags, and they are what needs to be replaced, get them from Arnott. I would replace both of them, and consider getting a spare if you do much off roading.