I havent seen adjustable ones for reduction box lately. Bus Boys may still have them on their web site, have a look there. They are great IF you need to adjust the suspension often like you mentioned. Otherwise they dont really offer any benifit. Once you have set the ride height if your car you rarely ever change it. With some time and a little effort you can dial in your stock spring plates just as good.
For the aplication you mentioned then they work well, you will need a jack to remove the load off the spring plate to make adjustments. Jack the van up, get out the allen wrench and adjust! That simple.
G.