OK - this is going to sound very weird, but here goes......!
If the thumping occurs when the brakes are applied, but not applied hard enough to cause any deceleration, then it is possible that the clutch which controls the planetary gearset is engaging, and locking the diff when it shouldn't be.
It is possible that the slight difference in the rolling diameter of your front and rear wheels could be fooling the TCU into thinking that wheelspin is occuring, even when it is not, and causing the planetary gearset clutch to engage briefly, to vary the torque split, or to lock the diff completely.
As before, since the thumping does not occur when the diff is constantly locked, I believe that the thumping is due to the clutch engaging at the wrong time.
.................I am probably completely wrong about all of this..........
Jason.
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1992 Charcoal Grey SVX - 165,000km
1992 Ruby Red SVX - 165,000km
....now with sequential VIN numbers!
1986 Holden (Isuzu) Piazza Turbo - 225,000km
1993 Mercedes W140 S500 Coupe - 153,000km
1990 Mercedes W124 300D - 313,000km
EG33 + 4EAT + VTD AWD = SVX
