ASR fault when engine is cold

Twice in the past week or so the ASR fault light has come on my '95 E320 (135K miles) when the car had only been run a mile or so. Both times the
car had sat for a day or more (so it was completely cool). I noticed that just before it happened that the car had a slight vibration as I accelerated (almost felt like an out-of-balance tire). The fault light came on and I could feel the car slow (the ASR must have temporarily engaged the brakes and engine reduction) so I pulled off the road, stopped and restarted the engine. This cleared the fault light and all was fine the rest of the day. Driving at the same location when the car warmed up, it's as smooth as could be.
Any thoughts on potential suspects? Could this be a problem with the tranny or rear differential?
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