It may be the sensors getting dirty like the others said. When they are inserted into the hub, they have to have a bit of grease on them. Try pulling the sensors, they may just need some lube

my new to me '02 just did this pulsing/false abs shit when i went to CT. to pick it up,
i just unplugged the big plug from the abs unit under the hood to get it home, works just like any other non-abs vehicle i've ever driven
but it's on my to-do list, my buddy claims his scanner will tell us what sensor is acting up ?????

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'02 Ranger Supercab, 4.0l, 4x4
Magnaflow catback duals, SB cold air intake
16" Bridgestone Blizzaks, Bilstein HD's

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took my truck to the mechanic and he said it might be that the brake fluid is dirty and may have got something in the lines and is messing with the abs motor.. not buying that though

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06, 4.0 V6, 4 door extended cab, auto
3 in fabtech lift, 31x10.5 AT, 15x8 eagle alloy rims
alpine CDA-105 deck, rockford fosgate 12in P2 sub, Alpine MRP-M500 amp

Yea I don't buy it either. The ABS pump only engages when a sensor tells it that a wheel is "locked" in that event a normally open detent in the abs module closes fluid to said "locked wheel. The pump then sends more fluid/pressure to the wheels that are not "locked"
So even if the pump was plugged (which i can almost guarantee its not) that wouldn't explain why its falsely engaging in the first place.

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1999 XLT 4.0 4wd