The standard Bilsteins are valved wrong for the Disco. For that matter, almost all shocks are simply because of the weight, especially in the rear. The best way to go is to get a tunable shock so that you can change the valving to suit your truck and driving style.
I am using Fox 2.0's and they seem to work well. The stock valving was way off though so I had to change that, but it was only 30 bucks to change all four shocks and took less than an hour. The main complaint is that you have to have custom mounts for the top & bottom of the rear shocks and the bottom of the front. It can get spendy in the $1000 or so range by the time you are done.
The second best, and much cheaper, option is to go OME. Get some rear shock mounts from EE to accomodate the droop and some spring retainers and you are set. Make sure to get the firm OME's. Total cost on this setup in less than $600 (I think).
Whatever you do, I would steer clear of Rancho/ProComp (same factory I think). The biggest problem with these shocks is the finish. They rust very quickly. They are also just prone to more issues than most good shocks and issues is not what you want to be dealing with on the trail.
Good luck.
