Sled work is actually a great form of single-leg or unilateral training. I know what you're thinking: sled pushing isn't a lunge, split-squat, pistol squat, 1-leg RDL, or step-up – so it can't be a single-leg exercise, can it?
Well, watch someone push a sled and – as is the case with sprinting – there are always points in time where only one leg is in contact with the ground.
So you could make the argument that sled pushing is the most basic, stupid-proof single-leg exercise out there, yet it's still quite effective.
It teaches hip separation (one hip extends while the other flexes).
It allows you to load someone without making them sore, as there's no eccentric stress.
It's so basic that anybody can do it – no matter how uncoordinated they are.
It can be used for everything from metabolic conditioning, to speed training, to strength work.
You can push it (high or low setting), drag it backwards, or do side sled drags.
You can do farmer's walks and pull it with a harness behind you. The options are near limitless – but regardless of what you do, you'll still be in single-leg stance. It's a beautiful thing.