Why does my 2002 ford explorer swerves side to side in the rain?

Why does my 2002 ford explorer swerves side to side in the rain?

How much tread do you have on the tires? Are they inflated to the proper pressure? (The number in the truck's door frame, not the number on the tire sidewall.)

That is a rear wheel drive vehicle (unless it is 4wd or AWD) and they are more prone to fistailing or hydroplaning. As mentioned above, tires would be a likely cause.