Meanwhile I came across this post in another forum:
how many miles are on it? i've seen the differentials go out on the automatics a little after 100,000 miles. i have a 99 sephia in my yard awaiting a new differential and my 2000 sephia that i drive on a daily basis was showing signs of a bad differential when i pulled the transmission and swapped in an manual trans. there was a lot of play in the gears and if you jack the front of the car up to where both wheels are off the ground and rock the wheel forward and and backward, you can feel to see if there's too much play in the gears. i heard a small clicking sound in mine, not loud but you could hear it. ...
At least the Transaxle on the "KiaStein" reached 170,000 Miles and the transmission itself seems to be alright, still shifting smoothly and flawlessly; the problem seemed to be the mere Differential, but couldn't be Sure 'till I Drop the Transmission and open it to carefully check its internal parts.