100% DO NOT flat tow any vehicle unless it has provisions specifically for flat towing. The reason for this is that the bearings inside the transmission will be starved of lubrication. When the engine is running and the clutch is driving the input shaft, all gears are spinning (even in neutral). When the gears are spinning, they pick up fluid and sling it into channels that feed the bearings with lube.
If a vehicle is moving in neutral with the engine off, the intermediate shaft is spinning inside the gears and no lubricant is getting into the bearings, however the gears themselves are not moving and are not carrying fluid for lubrication.
Vehicles with transfer cases can be flat towed because the transfer case itself can be put into neutral and therefore the wheels do not drive the output side of the transmission. Certain vehicles like Saturns had very specific provisions to lubricate the output side of the transmission.