My money is on the airsprings too. What year model D2 ? Mine is an 02/2004 purchased 06/2009 and about 6 months later it got saggy butt! 
I looked for every other reason it was leaking but it was the RH airspring as everyone said it would be. The soapy spray on water won't always show up a leak, and extending the rear to offroad height quite often causes the leak to stop.
The roll in the rubber in the base of the airspring is where they fail (the constant rolling motion progressively causes the rubber to delaminate and perish ... and as you drive the rear suspension is travelling up/down, causing the roll in the airspring to expose/close the air leak which is why it appears intermittant.
So I changed them both out (used Dunlops, took out Conti's) at around 5.5yrs / 125,000km. I just purchased 2x new Dunlops to carry as spares as most travel in the D2 these days is towing a camper, and the current airsprings are about 5yrs old with approx' 60,000km done. They look good, the spares are as insurance! 
Rear Suspension - Island 4x4 - Specialists in Land Rover and Range Rover Parts and accessories for all models. UK and worldwide mail order.
(Dunlops about $100ea / Conti's about $140 ... + postage).
Cheers
Kev..
Going ... going ... almost gone ... GONE !! ... 2004 D2a Td5 Auto "Classic Country" Vienna Green
2014 MUX LST with fruit
2015 Kimberley Kamper "Classic"
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