What are they doing with the Union soldier in The Good, the Bad, and the Ugly?

Two soldiers passed with a stretcher right when they were ready to drop the explosives. So they dropped the stretcher proceeded to pick up the solider. When the other batch left, you can see Blondie and Tuco look over their shoulders drop the leg and continue back to carrying the explosives to the bridge.

The point is to show that they were trying to blend in. Earlier they were running so no one really paid any attention to where they were heading. As they slowed to the bridge, it would have been within the view of the two soldiers walking from the river to see what they were up to. Thus, Blondie and Tuco tried to blend in by picking up the body. But they didn't want to take the explosives off the stretcher so they only pretended to move the body until the other soliders left.

Remember the point of blowing up the bridge was to force the armies to fight elsewhere. But seeing that the Union and Confederate forces were ordered not to destroy the bridge because of this (moving the armies), Blondie and Tuco moving towards the bridge with explosives would seem suspicious.