Rear-end going out how long will it last? swap ?'s??

Ok when i bought the stang the u-joints were shot and probably due to its death wobble the dam rear-end started leaking badly at the pinion seal.. Well i started noticing chattering when id take off and going around corners with my foot in it a bit.. stupid oil has a crapload of shavings in it! talked to a few ppl they all say get a junk yard unit and scrap it.. What im wondering is will i get any warning before it goes south the rest of the way?? i really dont want to have to dd my truck with gas so high.. but i dont want to get stranded eaither.. Also do auto rear-ends have lower gears?? if so wouldnt they be a good swap without getting ass gas mileage? i need the best i can get because my other car uses premium and gets 20mpg at best.. and my truck 16/17 at best.. mustang averages 25/26mpg.. Ive also noticed a recent loss in power and really wonder if the stinking rear-end is holding me back..

are 3.27s standard for auto's and 2.73's for manuals?? also would jumping up to 3.27s kill my mileage or be about the same with a tad more performance? I found a used unit from a 94 for $400 did all the gts have 8.8's they always ask when you call if its an 8.8 or 7.5... what would be the ratio on a 94 gt rear-end? it seems 5.0's were geared a bit higher?

Well i wondered because my t-bird came with 3.27's and the 5speed's were 2.72's i figured an auto stang's rear-end probably hadnt been beat on as much and would be geared a bit lower?

a 94 wouldnt have higher gears would it?? I dont want to make a step backword so how can you tell the ratios apart?