Rear end suggestions

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CudaFactHackJob said:
Yes. The joints are sometimes installed too tight. You can remove the driveshaft and flop the joints to make sure they move freely. But I think the problem is in the motor. Like a vacuum leak causing a lean condition to one cylinder or a bad plug wire or plug, or carbon buildup in one cylinder causing pre-ignition. An imbalance problem ALWAYS becomes more pronounced with the rpm of the part.
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Engine was just rebuilt and still being broken in. 210ish miles since been installed in the car. Plug wires are new (doesn’t mean one still can’t be bad tho).
 
3.91 If you had all the correct parts for the 360 swap damper/converter check the driveshaft/ u joints.
 
Having a similar issue as OP appear or become noticeable after going from 2.71-3.55 and adding sure grip. Found bad front u-joint. Replaced both. Checked wear pattern on gears. Good. Checked driveshaft orientation. Good. Changed out trans mount. Got somewhat better.Went from rubber to poly trans mount and got really bad so rubber mount is masking the problem. Gets worse at higher RPM like OP. Sound is like waaaaaawaaaaaaawaaaaaa. Another words not same pitch. Shaft looks good and both weights in tact. Question. Can a driveshaft go out of balance without damage? Tried the hose clamp trick at 45 degree increments and no change.
 
Ideally, if you have access to a lathe, you chuck it between centers, and using a dial indicator, you check the tube for runout.
When I didn't have access to a lathe, I made a pair of v-blocks with some angle iron and sealed ball bearings. I set the v-blocks on opposite sides of my 64 dart decklid, placed the driveshaft in the v-blocks, and using my mag base and indicator, checked it for runout. Ideally, you want only a few thousandths of an inch, but you could probably get by with 0.010". Incidentally, my professionally fabbed driveshaft for my pickup had at least 0.020" runout. I used a welding torch to straighten it better, after the local shop said that's as good as they get.
One other thing to check is the trans yoke engagement. If you don't have sufficient engagement/support the driveshaft will vibrate. My late friend, Brian, destroyed his transmission TWICE, by not having enough engagement and powering thru the harmonics until it exploded.
 
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