replace the vss/abs sensor on top of the differential. from memory undo the electrical connection there is 1 10m bolt, take the bolt out and pop the sensor out CLEAN ALL AROUND THE AREA VERY WELL. puts some oil on the oring and pop it back in. on the 98, 99 and 2000 the abs/vss in the differential controls the abs and feeds info to the pcm which then screws up the shifting on your auto tranny. Do you have 2wd?

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2004 Ext Cab, 4WD, 4.0, Auto, 154k. (4.0 engine is out of a 2009 Ranger w/96k) ( as of 7/1/2017)

2004 Ranger, Ext Cab, 2WD, 3.0, 5 speed , 4WABS, crank windows, no cruise, Edge 116k.(6/5,2014) (SOLD).
1999 Ford Ranger, Reg Cab, 2.5, 5 speed, 2WD, no AC, 150k (SOLD).