rear wheel wobble

Put the bike on a stand, hold the rear of your bike with one hand by the seat. Then take your other hand and see if you can move the wheel (grabbing it by the tire) side to side at all. If you can move that thing side to side (has play in it). Then you are probably looking at replacing the bearings, which is not a hard job if you have the right tools.

The other thing based on your limited post, is maybe the rim is just out of alignment and needs to be trued.

Couple things:
-Could be a bent rim
-Is the axle nut properly torqued (I had a freind with a KX250 complaining of a wobbly wheel and I reached down and the axle nut was only hand tight, torqued it properly and no more wobble
-If you grab the wheel and rock it side to side, do you feel play in the wheel hub? If so may be bearings.

I just picked up a 2004 YZ250 and a vital member suggested going to Thumpertalk for some really good technical suppoort. I checked it out and must agree. Almost anything you need to know about the bike can be found there. I agree with the others, sounds like bearings and possibly more... So to bike specific, yamaha, 2 stroke... Good luck!

Since it has a few broken spokes I would bet most of them are loose

As far as the seized spokes go, sometimes you can break them free by holding the spoke it self with a vice grip and then spinning the nipple. Other times, you are forced to replace the spokes.