The only reason you have to disconnect a prop shaft,or half shaft when being towed is because you have no lubrication to the mainshaft.
When the engine is running the layshaft is in constant motion and throws oil up on to the mainshaft,syncros and shift forks.
When the vehicle is towed the mainshaft is turning due to the prop rotation but it has no splash feed from the layshaft because you have no rotation from the input shaft or a gear selected.
The layshaft sits in the oil bath at the bottom of the gearbox and is the only supplier of oil to the rest of the box.
To tow a truck a short distance without disconnecting the prop either run the engine at tickover or hold in the clutch and engage a gear.
Both will keep the layshaft turning and splash feed the mainshaft.