pickup trucks are designed to carry heavy load at the back and can be RWD or 4WD. RWD is significantly cheaper and lower maintenance hence attractive. However an unloaded RWD pickup has next to nothing weight-wise on the rear wheels and I would imagine it is extremely slippery in the wet to the point of not driveable. How true is this? Do they normally come with electronic stability control? How effective is it in the case of an unloaded RWD pickup in the wet? I think US made pickup trucks are quite different and therefore not discussed here.
No matter if its US made or Japanese Made, a RWD pick up truck usually has 65:35 weight distribution , unloaded, they will tend to fishtail in the wet if not driven carefully.
I will not say most vehicles are "Undriveable" a light foot can always be helpful otherwise manufacturers wont sell RWD pickups.
The solution to this issue is to buy a vehicle with traction control or buy a 4WD pick up or load the back with sand bags so that there is some weight on the rear axle.
Vehicles like the Toyota Tundra have Traction control and LSD with computer aid to monitor wheel slip. To answer your question the TCS reduces the power sent to the rear wheels on detecting wheel slip. The LSD helps when one wheel starts rotating faster than another, as it locks the axles together so they rotate together, theoretically giving more power to the wheel that's not losing traction.
Other solutions is to check if the tyres have good treads if the the tyres are bald then you will fishtail a lot and then the "undriveability" comes in.