You might want to disconnect the bottom shock bolt, remove the wheel, and take the weight of the trailing arm off of the spring plate useing a smaller jack. Just makes it easier to turn the nut. The locking nut and adjuster nut are both torqued to around 180; if they haven’t been cracked loose before they may be a bear to loosen without a long breaker bar or impact gun, and the wheel would just be in the way. I put the rear up on jack stands supported under the torsion tube. Use paint or a marker to scribe marks on your plates so you have a reference point from where you started. Nut on the right is adjuster with reference marks.
Watch the smaller plate move against the larger side. Large plate goes up, ride height goes down. Large plate goes down ride height goes up. You might get ¾ inch travel total on the plates which might equal a inch and a half in height. You don’t even turn the nut a full turn. You can sit there and turn the nut back and forth and you will see the plate moving. Don’t count on much more than an inch or so with the adjuster. If you need more adjustment you will need to re- index the torsion bars.
You will definitely need the proper wrench to fit between the spring plate and tub or you could make your own by grinding one down until its thin enough to fit.