ASD Function Indicator On At Speed

[Old Barn Guy]

I have a recently purchased 1995 E300D with 125K miles. It runs very nicely. However, the yellow indicator light with exclamation point on the speedometer (not the lower one next to the glow plug light) for the ASD comes on when I hit 85 mph. Lose a few mph and it goes out, only to come on when speed goes up again. New, balanced OEM-sized snow tires on OEM rims all around, inflated to 36psi, on dry roadway. Hydraulic reservoir at D/S front filled to well above minimum level.

Does anyone have any ideas as to what might be causing this? I'm assuming at this speed, the locking feature on the diff does not function.

Any ideas or suggestions most appreciated!

Regards,
Tom
Closter, NJ