The torque tube is the tube in which the drive shaft runs to the rear differential. In most vehicles, like a hotchkiss drive, the engine twists against the frame or body to spin the drive shaft. The drive shaft then attempts to turn the differential. This means that engine torque tends to lift the right tire and push down on the left tire. It is why a car with an open differential always spins the right (passenger side) tire on takeoff, since engine torque is twisting the whole axle assembly.
The torque tube instead, mounts directly to the differential and to the transmission, so that no twiting force is applied to the axle or the frame by the engine. Instead, the engine twists the shaft inside the torque tube, which then transfers the torque right back to the transmission case. The rear axle, then, does not attempt to twist against the body under power.
You could change your car over to a Hotchkiss drive, where the engine torque is transferred to the body, and the rear axle reacts to engine torque by attempting to turn, causing weight distribution change side to side. But you’ll need different engine mounting, different springing, change to a 4 link style suspension in the back, and other changes, as well. I would NOT suggest doing that. Instead, if all else fails, a very good machine shop should be able to repair or fabricate a new torque tube. Likely yours is damaged by hitting something, or a failed drive shaft or joints hitting it.
It is complex, heavy, and expensive, but it was done to spare your car from twisting the frame or body from engine torque, and to improve cornering characteristics and prevent “under power” handling changes.
Actuallly the bushings and donut are worn out. They are on the way so I was just intrigued to see what Freepers though of the reason it was there. I plan to replace the drivetrain with either an Ecotech 2.2 (an Opel design used since 97 in Europe) out of a rwd Pontiac Solstice or an injected 2.4 Liter engine out of an early 90’s Opel Tigra. Bored and stroked, would be a great ride. At that point, I will reconsider the transmission and rear drivetrain options. Input here has been great by the way!