Where is this going? On a floor where you can get to it though the ceiling from below? Or, in concrete?
Hopefully, the existing vent line won't be too far away when you move the drain line.
It depends on what you plan to do around the tub/shower. If you are going to tile it, then you'd need to take up some of that 2" with cement backer board. You could put some shims on the walls, then attach a vapor barrier (either plastic or roofing felt), then the cement board. You could cut shims out of 1/2" plywood. That would make the 2" you'd need, then the tile would fit over the tiling flange (does the tub have a vertical flange on the sides?).
The tub normally would need to be up against the walls, so you could use two layers of that 1/2" plywood on the studs on the ends.
To get the tub level, once you have the drain moved and ready to attach, you could make some deck mud (5 parts sand to 1 part portland cement - you can often buy this prepared in a bag at a tile store, then mix it with water).
Spread this stuff out on the floor under the tub, thick enough to support the bottom of the tub. THen, smush the tub down into it carefully until it is level. Make sure it is level in both directions. Then, leave it overnight. It will then be fully supported and level, ready to complete your install. Normally, you also want to install ledger boards under the rim. The height of those can be determined, and then installed prior to setting the tub in place. It will give you a good guide as to level. Screw (preferred) or nail them to the studs at the proper height.
