There is one differential wear mode that you can test for with no disassembly. With the transmission in neutral reach under the car and turn the driveshaft back and forth to observe the drive train slop. If the driveshaft turns very much back and forth with the car sitting still then you may have a badly worn differential. This is also obvious in those cases where you put the car in park and it will still creep forward or backward a good distance before settling against the transmission stop. Same likely reason - differential wear.

If its not doing this then the next likely wear point is probably the bearings inside. These really should last a long time with proper lubrication, but I suppose there are always exceptions.

Personally, I think if you have little drivetrain slop as described above, you should drive the car until something else under there goes wrong, like the axle shaft boots or something. Then consider a differential replacement as part of that repair.

400K miles is nothing for a well-maintained differential - especially if the transmission is shifting properly (not too hard) and the driver is moderate.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles