With those vehicles you are probably looking at a max of a 1500-2000 lb trailer, but it depends to some extent on the engine and rear axle they have and what else you will carry in them. The Ranger with a V6 is rated higher, up to around 3800-4000 lbs, but I would stay well below that figure.
Minivans are not great tow vehicles and older ones like your Caravan are often tired anyway.