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Let me guess, you replaced ball joints recently, right? Or may be you replaced front rotors and installed new one for non-4WABS Ex (pre 93)?
Anyway, first of all I'd checked front sensors' resistance (should be near 0,8-1,0 KOhms if I still remember correct -- hope someone could correct this if I am wrong).
Then I'd see if rack on back-side of rotors is OK and is not dirty.
The most common reason for such behaviour is damaged front antilock sensors/racks. I'd rather do not consider rear antilock sensor being the contributer. Also it cannot be damaged wiring as startup ABS test seems to pass OK.
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