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I can't absolutely say you will damage the tranny if you don't disconnect the driveshaft. But towing with the D-shaft connected means spinning the OD output flange, the OD innards, and the tranny mainshaft (aka output shaft). This always happens when coasting. (If it didn't, we'd get no engine braking.)
But the tranny's Input shaft doesn't spin, since the engine is not running. This means that the Idler Gear (aka Cluster Gear/Layshaft) won't be spinning either. And it's the spinning Idler Gear that normally provides the splash lubrication inside the tranny. When towing with the D-shaft connected, there will be NO LUBRICATION for the spinning mainshaft, its rear bearing, and all the needle bearings or bushings for the 1st, 2nd, and 3rd speed drive gears, synchro hubs, etc.—all riding that spinning mainshaft—with no oil splashing.
Others will say they've towed RWD cars without disconnecting the driveshaft and suffered no damage. I'd say no apparent or immediate damage. But what about possible shortened bearing/bushing life? It's not a gamble I'd want to take, when I can have the driveshaft uncoupled and tied to one side in 1/2 hour or so.
From MotorHome magazine's web site:
Q: Why is it that a vehicle with rear-wheel drive cannot be towed with all four wheels down?
A: Nearly all non-4WD rear-wheel-drive transmissions will be damaged if they are turned by the driveshaft for long periods without the engine running to circulate lubricants within them. Therefore they are not towable for long distances or at highway speeds.
From Pensketruckrental.com (Dolly towing instructions):
12. Disconnect the drive shaft on the towed vehicle if rear-wheel or four-wheel drive. Consult your dealer if you are uncertain of the above. Towing above vehicle without disconnecting drive shaft will damage towed vehicle's transmission even if transmission is placed in neutral.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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