What would cause this?...?

hi, i have a 1996 ford explorer. and my left rear axle seal has been seeping for quite some time and havent had the chance to put in a new one. but now there is a squeaking noise heard, and a vibration when accelerating. it might be my u joint because it has been going for quite some time as well. but i just found out today that my left rear wheel has some in and out play in it. (like a plunging effect) i was wondering what could be worn out to cause this problem?