99 S10 Blazer brakes pulsating

Does the pulsating feel like it's in the body or in the brake pedal? Do you here any noises associated with it like ratcheting or similar?

It sounds like the anti lock is activating. Are you on pavement when it happens or gravel, sand, etc?
Is the anti lock brake light on in the dash?

How about when you first start the truck. Does the anti lock light come on and then go out after running?

You may have a sensor problem since it's not turning the light on. Does the speedometer work?

Then it's probably a front sensor. You can try removing each sensor and cleaning the surfaces where it mounts to. Scrape any rust or buildup away. Also check the wire connector. These are known to get damaged. Replacement connectors are available at most parts stores if they are. If you have a DMM you can also test each sensor. Unplug the connector and hook the DMM to the sensor harness and check for at least 350mV while spinning the wheel. Anything less and if it's after you tried the cleaning then it will probably have to be replaced.