Re: 2002 Mountaineer Differential/??????
I can't say if you'll make it anywhere. However, I'd check that you're not driving with the axles locked (4HI engaged) and that it's in fact AWD or 4 Full Time mode. I would assume that you have a bad axle. If gears are properly maintained they'll last forever (for the most part).
If it's a chattering sound (like you had mentioned; metal on metal) that increases with speed, then you more than likely have a problem with at least one axle. Be sure to phone the shop, ask to talk to the guy who did the work and see if they found any metal pieces in the rear axle when the fluid was changed. This is assuming that he removed the cover, instead of sucking out what fluid he could get out and charging you full price for a half-as$ed job. If he did find metal, other than fine metal dust on the magnet then the clutch packs can become loose in the pumpkin and grind together depending on wheel rotation.
If your parents have a shop with a vehicle lift, put the vehicle on the lift, and put the transmission in to park and make sure it's a frame lift, and that the wheels are off the ground. Now, turn the wheels. The wheels will all turn in different directions, so don't be concerned with this as it's normal for AWD vehicles. Try turning each wheel at a good pace, and see if you hear any noise. If you do, then you can narrow it down by getting a relative or friend to turn the wheel while you pinpoint the source of the noise.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!