My 99 Land Rover Disco II is sagging in the rear end. What will it take to fix it?

I am assuming you have the air suspension in the rear. What you need to do is go on Roversnorth.com, and order the air to coil spring replacement kit. Fairly straightforward install, get it done ASAP.

Probably something that has to do with the Air ride suspension. they use air bags to adjust ride heighth per different settings. probably leaky airline, or leaking bag. take it to a shop to have it professionally checked or you might be spending a lot of money throwing parts at it to try and fix it yourself.

You might need new shock or strut or new leaf spring

sounds like you need new shocks .you can replace them yourself if you look at the new ones its prety self explanitory