If you want the ride height where it is then getting a stiffer spring is about all you can do. If it's squatting too much, what else can you do to correct the problem but stiffer springs? The new shocks could help but in the end you need the rear to not go down as much. Opening up the drive shaft tunnel or making the drive shaft from a smaller tube will stop it from hitting but it still doesn't stop the rear from squatting too much. It will still squat too much with the rear of the car going down too much, the drive shaft just won't hit anymore.
So figure out what you want to do, correct the problem, or leave the problem and make it work by giving the drive shaft more clearance.
I am not being a smart aleck here, if the squatting isn't that bad you may want to live with it and simply create a little more drive shaft room.
But understand it may look like it is "barely" hitting the tunnel and you "relieve" the tunnel with a big hammer and you find out it is still hitting, then you relieve more, and it's still hitting.
If you are fine with a little more height in the back then some spring spacers under the spring and raising the back of the car a little may do the trick. But what happens when your BIG friend gets in, it's going to hit again!
Is the rear end moving from side to side when the rear of the car goes up and down? Did you change the rear suspension at all or is it original? The rear end could have a locator bar that is at an angle and the rear is going over to one side when the rear end goes up? Take a look at that too.