99 ford mustang rear end noise?

First a little side play is normal, the clicking is probably gears in trans. normal when in neutral and turning drive shaft. If you turn yolk on diff. should only move about 1/6th of in. before axle begins to turn, more than that ring gear and pinion worn out or needs re-adjusting. You need to replace seal, brg.is probably pressed on axle, You can take to Napa or shop and get old pressed off and new pressed on best way over chisel. I'm presuming you know how to remove axle. If not you can e-mail me.

Im sorry but it was difficult to read and dicipher what you wrote, because you do not know the name of the parts you are working with, what you are looking at, and you cannot describe it because of this. It is very important, that you know what you are doing. And irreguardless you should have a shop manual before ever attempting what you are trying to fix. NO disrespect meant, please understand, it is difficult enough to try to guess accurately, but impossibel to help if you cannot name the parts correctly. Buy a shop manual for that vehicle. and read it, familiarize yourself with the parts, the names and what ever else is in there you may need it soon. As to what you wrote, Im assuming you found some play. if it was in and out on the axle, and there is a leaky axle seal, stop driving the car, and get the wheel bearings checked by a professional. A bed wheel bearing can cause a lot of damage to the axle , and grind it into bits, not to mention if the rear end runs out of gear lube. (the rear end self destructs. ) in order to replace a wheel bearing, you have to take the rear end apart from the inside. Its a shop job, more labor intensive than anything else, I just had the wheel bearings (and axles unfortunately) to the tune of six hundred bucks.

But 400 of it was parts, both axles needed replacing. Better safe than sorry, and by the way if a c clip were to come off the wheel and axle would slide right out of the housing, so dont worry, but get the axle bearing replaced if necessary before it gets greasy crap all over the back breaks.

Why? It would be so much easier to buy a Mustang II with the 302 already in it - many more of those are still around than the ones with the 4 & V6s, anyway. You will end up putting way to much time & money into this project than it is worth - believe me. Been there, done that to a '76 Mustang II, swapped the V6 for the V8. It was a major headache, then I gave up on all the hassles and bought a 1979 Mustang Ghia with the 302 already in it. Good choice! Drive the 'Stang they way it is & save up your $$$ for it's repacement.