Rear end trouble.

Hmm no one has replied? I had this problem it turned out that my shocks were not tighten all the way (play in the rear end) but never took the wheels off to check the axle play.

I would pull the axles out and see if everything looks normal if I were you, and go from there. The axles will have a little bit of play, side to side. But not 1/4 inch.

Check all your rear end mounting points. Shocks, aswell as upper and lower control arms for broken bolts, bad bushings, ect. ect.

Okay well, when the wheel is off, there is nothing holding the brake drum on except the shoes if you have the parking brake on. So the drums moving in and out is perfectly normal. If the axle is moving 1/4 inch in and out with the drum, then yes, that's too much.
Pull the drum off and see how much the axle is actually moving in and out. It shouldn't be more than like a millimeter or two. If that checks out, save yourself some trouble and check everything else for the cause of the slop before cracking into the diff.

Sounds dumb...but,you sure it's not the float inside the gas tank? If you have a quarter tank of gas or less and you push the back end sideways to make it shake from left to right,the float knocks against the edge of the tank.

And being that your tire is wearing excessively on the inside,it sounds allmost like a straight shoot to either a loose or damaged shock mount or control arm (What he said above).

You didn't mention if it was just on one rear tire or both sides.Also,if your going to be taking off the drums,i'd seriously buy a set of new drum brake cylinders.I noticed when I was looking into rear end noise once on an 89 GT I had,upon removing the brake tensioner springs,the piston within the cylinder just popped itself all the way out.The rubber seal was old and eroded and fluid was spilling from the cylinder........Get new ones.They are only $10 bucks a piece.And i'd also highly reccomend you get new brake pad tensioner springs also.They get so stretched and pulled apart.Do this unless you plan to convert to disc brakes.