Occasional tugging on right side of 88 Silverado??

This only happens in wet or long grass, never on the road. At low speed 5-15mph I get a sudden yank on the front right side like I ran over a 6-10" stone or a brake suddenly went on and off real quick. I can't see anything wrong with the brakes. The pedal doesn't pulse nor are they low. I did have that front drive axle replaced last fall but its been ok before and since. Truck has 160 K and is is mechanically sound condition. Any ideas????

does it have abs brakes?

It has them only on the fronts on that model. Seems dumb but they work quite well. Is it time to change the brake fluid me thinks? I have been meaning to ever since buying it 2 years back.