Check out the video I linked in my answer to a similar question at the Engineering SE. In that answer I said that the problem with trying to use differential steering in a wheeled vehicle is that you have to accept a tremendous amount of slip in order to perform turns.
In a wheeled vehicle on a hard surface, like pavement, this means burning tires to make common turns. Tanks don't have this issue because they aren't designed to and typically don't operate on roads, they operate on soil. In that scenario the ground gets torn up instead of the "wheels"(tracks). However, with a tank, there isn't as much damage to the ground because the tracks distribute the weight of the vehicle over a much larger surface area, reducing the ground pressure it exerts.
The problem you'll have with a differentially steered wheeled vehicle operating in sand is that you get small points of contact between the tire and terrain due to the circular nature of the tire (point of tangent plus some tire/terrain deformation), and that small point of contact boosts ground pressure. Then, on top of that, you're going to spin tires - you're asking for the vehicle to dig itself in.
Your vehicle is light, so if you manage to get some large/oversized tires and run them under-inflated you might be able to get by, but this for me is one of the great joys of engineering: the best, fastest way to get a satisfying answer is to try it. Try what you want to do in the environment you expect to operate. If building a prototype is too costly/time consuming I would choose a different method of locomotion for fear of digging into the sand, but if your actual terrain is "sandy" instead of "sand", as in by a river, or if the sand is wet or compacted, you might be able to get by.
Finally, that said, if you really want to go this route and weight is an issue, I would use belts or chains to attach the wheels to the motors. Using a gearbox would be expensive weight-wise and would also probably limit the wheel base you could use.