Just in case you're not seeing it..... As others have mentioned, it appears your car is missing the TORQUE ARM which runs along side the driveshaft. This limits the twist of the axle (or up and down movement of the u-joint) while under acceleration on the Monza or '75 and later H-bodies with the 2 lower control arms and the Panhard Bar....
This is known as the
"3-link" style rear (
2 lower control arms +
1 panhard bar =
3 attachment points).
or "Torque Arm" style rear. The Panhard bar only centers the axle left and right in the body.... it isn't supposed to limit limit axle twist.... which seems like it's the only thing keeping yours from completely flipping backwards.
Earlier H-bodies use the
"4-link" style which use the same style 2 lower control arms, AND also 2 upper control arms to limit axle twist (u-joint moving up and down).
2 + 2 = 4. It does NOT use a torque arm nor does it use a panhard bar. The 4 control arms keep the axle centered left and right as the two upper arms are angled inward/outward.
I've attached a pic of what your car should have, since it has the panhard bar, and no upper control arms, it MUST have a torque arm.... there is nothing to keep the axle from twisting up and down.
When the driveshaft turns the pinion shaft/gear, it forces the ring gear downward (causing the axles/wheels to turn forward), this downward force on the ring gear, in turn forces the rear end housing to rotate UP at the u-joint in an arc...... the torque arm keeps it from twisting too far up, binding the u-joint and causing vibration.
I hope this helps...... looks like you need the torque arm.
Monza 3-link Top View illustrated.jpg
I've also attached a youtube video of what you are probably experiencing..... the axle twisting under acceleration or load.... it's called "axle wrap".
To many of us in the off-roading and 4-wheeling community where our Jeeps are put under heavy load (throttle from a dead stop) to get over logs, stumps, rocks, etc... causes the leaf springs to flex or wrap .... which allows the axle housing to rotate up.... snapping off the u-joints and ruining the pinion ears or driveshaft.
Notice how the axle rotates upwards at the u-joint when he puts it under a load at about the 1 minute mark .
http://www.youtube.com/watch?v=Ny33NtNdFOEThis one shows a weak 4-link set up failing.... causing the axle housing to rotate to extreme... which breaks u-joints and such. You gotta have a torque arm.... you don't have anything to keep it from twisting.
http://www.youtube.com/watch?v=08UIvr6SotkThis is what a torque arm does... they call it an antiwrap bar..... yours is missing.
http://www.youtube.com/watch?v=Ef6lTenmPUo
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