It's going to take a lot more than a switch. However, I would advise only using diff lockers when your speed is less than 20 MPH, because they affect the turning radius and the handling of the truck. As an alternative to getting either type of diff locker, you can try carrying cat litter in your emergency kit or using tire chains, depending on your situation and if chains are allowed.
Inter-axle differential lockers lock the drive wheels on the left front and right rear on most Volvo trucks (though sometimes it is set up for the right front and left rear instead). The wheel differential lockers lock both rear wheels together. Wheel lockers are always better than inter-axle lockers when given the choice, and if you can get both a wheel locker AND the inter-axle locker, that will help even more by getting power to 3 out of the 4 wheels. I would suggest consulting your dealer or shop for advice on which ones you should spec for your situation.
Also, if it's just happening at the loading docks, try easing up on the clutch and slowly accelerating (instead of putting power down) to help the drive tires gain traction. This technique has helped me get out of some sticky situations, including getting out of a small snowbank yesterday, without the need for a tow. (I here in Illinois, USA don't get as much snow as you Canadians, but I still get some. Think of it as a dusting vs a heavy snowfall, but I still get stuck in some situations).