is it only wobbly front to back, or is it wobbly in all directions (like a chair with a short leg) . if this is a prebuild stand, it should be almost perfectly level... it may be your floor.
my tank is on carpet and will rock front to back slightly if you push it. A small amount of movement can be expected if you are on carpet since the whole thing is VERY top-heavy, and you dont have a direct interface between the stand and the hard floor.
check everything is level... if it is off a bit, make everything as level as you can get it. shoot for perfectly level, but a tiny bit off probably wont ruin anything. my tank has about a 1/16" difference in water level front to back, and i have no problems with my tank leaking or breaking.
also, if you are on carpet, you may have part of the stand resting on the tack strips. make sure you are a few inches away from the wall so you avoid the nail/tack strips.