Wheel locking on gravel!

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.......

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

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