have a similar problem with my 99 impreza, seems to be eating rear diffs like their candy. breakdown of my ordeal.... bought the car as a 99 TS auto. blew the transmission, bought a rotten 99 impreza wagon 5spd for $300, exelent deal considering the motor was rebuilt, pulled the motor transmission rear diff and wiring harness, dropped it all into my sedan, changed all the fluids, drove the car for 2 or 3 weeks, chewed out the rear diff.... ok i can understand the first 1...... car had sat for a few years before i bought it..... bud rebuilt the motor then parked it a month later cause the car rotted out...... so sure gear not moving rusted up a little..... swaped the diff out for one out of a 98 impreza wagon 5spd, and here i am 2-3 weeks later, diff chewed out again... i have a 99 forester but have no idea what the gear ratio is, cause i have noticed that its a 1st gen trasmission and a 2nd gen motor. 4 bolts for transmission, block can take 8, from what i've read everywhere the gear ratios for both donor cars are 3.9, all the other diffs i have seem like they should be 4.11 and i'd shit myself to find out the forester had 4.44