If your truck has a difficult time going up-hill in the snow or tens to spin-out under slick or web conditions, chances are you need more weight in the back? And adding a few sandbags in the truck bed, centered over the rear axle is often an easy, affordable, safe and effective way to improve a truck's stability and traction. (Any farther back than over the rear axle -- such as up against the tailgate -- may raise the front end of the vehicle and cause additional handling difficulties.)
SAFETY NOTES: 1) Adding trunk weight will not improve traction when driving on ice and 2) adding weight to the trunk is not normally recommended for Front Wheel Drive, Four-Wheel Drive or All Wheel Drive vehicles. These vehicles already have a great deal of weight over the drive wheels that require the most traction.
It's best to check your vehicle's owner's manual for the safe vehicle payload. (Too much weight can compromise your vehicle's safety, stopping ability and fuel efficiency.) As a general guide, start with 250-300 lbs for 1/2 ton pickups or 300-400 lbs for 3/4 to 1-ton pickups. From there, you may find that you need more or less through trial and error.
Sources: https://blog.tiresplus.com/driving/adding-weight-truck-bed, https://www.lesschwab.com/article/does-adding-weight-in-the-back-improve-my-cars-traction-in-snow.html