Yes you can tighten the torsions up and gain a little lift .But you will sacrifice ride and might have problems with alignment as well.
If you are only looking for 2 inches I would pick up a set of torsion keys .
Here is some good info for you .
http://www.fullsizechevy.com/forum/...keys-torsion-bar-cranking-replacing-tech.html
Horrible idea, listen to this guy below
Dont spend the money on torsion keys!!
They are no different than cranking your stock keys. They just allow you to tighten further which you don't want to do.
It's a misconception that torsion keys help alignment, don't kill cv joints, balljoints and ride better than just cranking the t bars. In order to gain lift with torsion bars you either have to lower the frame pivot point or add preload. Adding preload is what you do when you crank the stock keys. So why pay for aftermarket keys that do the same thing as stock ones the just let you make it even worse.
As he said, the leveling keys only allow you to crank farther, they are simply reindexed keys that allow you to keep cranking where the stock bolt would have bottomed out. They still have the same effect on alignment, ride, and angles.
The only true "useful" leveling kit out there is the Cognitio kit that includes new upper control arms. They are slightly longer allowing easier alignment, they correct the upper ball joint angle (which is more severe than the lower), and they replace the metal/metal contact of the droop stop with a urethane bump stop for a less harsh downtravel. You still have horrible CV, tie rod, and lower ball joint angles. Ride still sucks due to the LCA being at a sharp angle. It is a common misconception that cranking the bars itself makes it ride stiffer, the actual cause of the stiff ride is the angle of the LCA is steeper, effectively creating a shorter moment arm on the torsion bar. Think of it as trying to turn a bolt with a ratchet rather than a long breaker bar.
I have gotten roughly a max of 2.5" out of stock keys, but every truck is different, some I haven't been able to get nearly level (mostly 1/2 tons) and they did require keys, or stiffer replacement bars from a 3/4 ton. I have never had any issues getting a proper alignment even with 3" on a set of keys I did for a guy, but I was not happy with the angles and I made that clear when I was doing it for him. I have never ran anything higher than 1.5" of crank personally simply for the ride quality and improved angles over going higher.
To the OP, YES jack the truck up to take the weight off the suspension, otherwise you may twist off the adjustment bolt, I have heard of many people doing this. Also get an alignment ASAP as your camber and toe will be way off, but an alignment will fix that right up.
If you realllly want leveling keys I have some I could sell for $40 that I removed right after I bought my truck. (I had installed the keys for the previous owner.)
Just to help explain mine/MarsFab's point, here is a comparison shot of torsion bar keys.
The one on the bottom is a stock 2500HD key, known as a "purple key" due to a small purple paint mark used to distinguish them at the factory, that mark usually wears off soon, this is also factory installed on the Silverado SS to lower it since putting a 2500HD key on a 1/2 ton will lower it you will see the opposite is true of the vice versa below.
In the middle is a leveling key for a 2500HD, (which is nothing but the stock key from a 1/2 ton z71, also known as a "green key", this allows you to crank 2" farther than a stock 2500HD key, all of the expensive leveling kits you see are nothing but a copy of this casting and priced 3X as much. You can pick these up at the dealer for $60 brand new, or get them from a junkyard)
The top key is a leveling key for a 1500, (which is nothing but a stock Ford F-150 key, which just happens to fit and lift a 1500 GM 2".)
As you can see the only difference between the keys are the indexing of the hex shaped hole, cranking the adjustment bolt would have the exact same effect as swapping these keys except they give the ability to go farther. (The "ford key" casting has the triangle cutout, but many aftermarket keys are the same index without the cutout)
I usually charged ~$30 for installing keys depending on how rusted/stubborn the t-bars were in the LCA/Keys, although you are still looking at $50-80 on top of that for the alignment. 