Wheel locking on gravel!

[bootleg233]

Was wondering if anyone has had this occur?: A friend of mine has a 2005 GMC Canyon 4x4 and is having problems with the brakes on it BUT only on gravel roads. Seems that 1 (one) wheel on each axle locks up like the abs is locking or something 1 wheel on the front and 1 wheel on the rear. Anyways I was wondering is this a sensor problem which seems odd 1 wheel on each axle but I guess anything is possible and was wondering about anything ya'll had tips tricks whatever if it is the sensor about replacing it. Thanks in advance.......

[bootleg233]

No one else having brake problems i reckon????????

[maxwedge]

To see if this is an abs issue a scan with an abs scanner will show sensor inputs under the conditions.