1991 2.9 4 speed My rear anti-lock light is always on and recently at random times when I slightly touch my brakes at a normal rate like I usually do to slow down the rear will just lock up? Anyone know why?

 

A good place to start would be to do a thorough visual inspection of the rear wheels, looking for oil or brake fluid contamination.

Lets look at the reasoning here.
If there is oil, grease or brake fluid contamination on the brake pads/shoes , when the brakes are applied the brakes GRAB and can even lock up.
The anti lock system sees this and causes the light to illuminate. Before getting involved in an electrical problem that may not exist, it is prudent to DO THE SIMPLE CHECKS FIRST.!

 

Try to remove the sensor and clean it. The sensor is located on the top of your rear differential.

 

Okay I’ll let you know what happens thank you!

 

Also make sure your red brake light on cluster proves out correctly.
1) when you set the parking brake do you see red brake light? If you do that means the bulb is good
2) if the bulb is good, next time you drive release the parking brake first, then you turn your key if you see the red brake light on when your key is in the ON position or you see the light when you turn the key to START, but the light goes away by itself, then it proves out correctly.

If you have both red brake light and yellow abs light on at the same time during driving that's a different story (check the fluid level) than just yellow abs light alone.
Anyway clean the sensor first, don't buy new unless you see significant damage, and see if the light comes back or not. Then we need to retrieve the flash code (error code) and see what the problem is, you can also reset the light there once you get the flash code.