Ireplaced both wheel speed sensors due to abs

Have you changed the wheel speed sensors? For the most pard the abs will engage at low speed due to a cracked tone wheel in the hub or cv.shaft

Bad abs sensor you can pull the abs fuse under the hood usually large amp fuse 40-60 amp abs wont work put brakes will function better till repaired

look at the front wheel speed sensors and look at the reluctor rings on the front shafts they crack and throw off the speed to the abs module

something as simple as a piece of string or trash can cause this, best to visually inspect the sensors, or do wha I do-go to one tof those carwashes that are do it yourself, and shove the high pressure wand (not too close) underneath near the inside of each tire, and clean the sensors off-sometimes a good drive int he rain works too, but it is not raining a lot here lately....

Here is a copy of a GM bulletin that applies to you concerns of low speed ABS activation.

Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface) #03-05-25-007 - (09/29/2003)
Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface)

1999-2004 GMC Yukon XL

Condition:
Some customers may comment on ABS activation at low speeds, usually below 8 km/h (5 mph). Upon investigation, the technician will find no DTCs set.

Cause
The cause of this condition may be an increased air gap between the wheel speed sensor and the hub reluctor ring due to rust and debris built up on the sensor mounting surface.

Correction
Measure AC voltage and clean the wheel speed sensor mounting surfaces.

Raise and support the vehicle.
Disconnect both the front wheel speed sensor (WSS) connectors at the frame and harness.
Place a Digital Volt Meter (DVM) across the terminals of each WSS connector.
Rotate the wheel clockwise approximately one revolution per second. The minimum reading should be at least 350 ACmV's. If the reading is less than 350 ACmV's, remove the wheel speed sensor. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual. Plug the WSS bore in order to prevent debris from falling into the hub during service.
Clean the WSS mounting surface on the hub to remove any rust or corrosion using a wire brush, or equivalent.

Important
Make sure that the sensor sits flat on the hub. Check the sensor flange against a straight edge to ensure flatness. If the sensor flange is distorted, replace the sensor.

Apply a thin layer of bearing grease to the hub surface and sensor O-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344.
Install either the original sensor or a new one in the hub. Ensure that the sensor is seated flush against the hub. Refer to the applicable Wheel Speed Sensor Replacement procedure in the ABS sub-section of the Service Manual.
Place the DVM across the sensor terminals and recheck the voltage while rotating the wheel. The voltage should now read at least 350 ACmV's.

Good luck and thank you for using Fixya.

most likely you have a wheel sensor going out. The only way to tell which one is with a data scanner that can access the ABS system.

It will see all the tires spinning at the same speed until one shows a different speed.

Also make sure you have all the same tires and the air pressures are the same.

It might be a coincidence and nothing at all to do with the impact.

Lots of modern cars have abs reluctors built into the wheel bearings. This is because the external type tends to be eroded by salt and water increasing the abs sensor air gap so at low speeds an output will be lost and the brain will assume a wheel is locked and deploy the abs.
Sometimes a scale and corrosion build up will split the reluctor ring.

Before seeking more complicated solutions it would be well to ensure any exposed reluctors, sensors, air gaps and wiring at each wheel is in good order.

Check the speed sensors at each wheel. http://www.ehow.com/how_4521137_replace-abs-wheel-sensor-chevy.html

Check your ABS sensor on your wheel hub. The ABS sensor may be going bad or needs to be re gaped. You can do the work your self to save you some money, Take the front wheel off and look for your ABS sensor, There is going to be a wire harness along your upper A arms, just follow it to the wheel hub. Give it a good spray with brake parts cleaner and re gap it closer to the speed ring on the wheel hub. If you don't have a feeler gap, you can use a dime as the gap reference. After your have done all that and you still have the problem, you can go to the dealer and have them look up if there was a recall campaign for the ABS activating during low speed stops. Good luck and keep me posted Be glad to help.

make sure all tires are same size and inflated to correct pressure