Odd Vibration, Driveshaft Inertia Ring?

[ojh]

I'm a newbie here trying to solve a vibration problem at about 60ish mph.  The is a restored '68 Charger RT, 727, 440 motor all factory stock with fresh transmission, new torque converter, new rear gears, new carrier bearings, new wheel bearings etc.  The original driveshaft was replaced due to a vibration, the replacement shaft cured the problem for a brief period.  The vibration returned, I pulled the new shaft and it is perfect.  I am now replacing the trans slip yoke and front universal to see what happens.  Reading the factory manual an 'inertia ring' is specified for these cars as part of the driveshaft at the slipyoke end.  There is a fair amount of discussion as to they are needed or not.
Any suggestions?
The vibration is felt thru seat of pants, not thru steering wheel or throttle.
I have replaced the torque converter, rebuilt the trans, replaced rear gears and all bearings in the rear, balanced and check tires for true, I've changed all but the ground it sits on and am missing something (all of what was done was needed irregardless of the vibration).

[daveco]

It's odd that the vibration went away for a time, with that in mind it seems that something must be moving or shifting. I doubt that the inertia ring (needed or not) is part of the problem.
Has the runout (with the driveshaft installed) been checked?

[moparnation74]

If you are going back to factory correct, than you would need a shaft with the inertia ring......I believe they were being phased out in early 69, my survivor 69 GTX 4spd built 10/68 has the inertia ring driveshaft...

If not going back to correctness than use a proper size, length, and balanced drive shaft.

Sounds like you replaced everything, so if that was done properly and seems fine....what have you not replaced etc..

How is the brake system?

[ojh]

I may have found it.  I looked at the original slipyoke and universal, the 'C' clip wasn't right.  I popped the clip off and the cup came out of the yoke without me doing anything.  It was an 'ah-ha' moment, when I excersized the universal it didn't feel smooth at all, it felt like some needles might be missing or mashed up inside.  When I removed the cups they looked normal, I was surprised.
I'm now trying to install the new yoke but the universals are giving me fits, they are solid and I'll get them almost together and since air/grease can't escape the last little bit is all but impossible, I'll assume I've got a bearing in there and take it all back apart.  This is my fourth time on it.
Any tips or pointers?
I'm hoping to road test today to see if vibration is gone, I'll let you know.

[Just 6T9 CHGR]

Have you checked your pinion angle?

[ojh]

Sorry about the delay, I had to replace a couple exhaust gaskets, troubleshoot the headlight buckets and an electrical issue.  I just took the car for a ride and it is all but perfect, vibration gone but it has to go back to the alignment shop. 
To summerize, the car had a vibration and the new owner replaced the driveshaft, the problem went away for a short time.  He brought the car to me along with yet another driveshaft.  When I removed the replacement driveshaft I found the universal joint connecting the slip yoke was improperly installed, it worked ok for a brief time but when I took it apart it felt like it had little pebbles in it.  That was why the problem was briefly solved and returned. 
Now I got to fix a 'bouncing' fuel level needle, road test for about a hundred miles and it is good to go. 
Thanks for your help, I appreciate it.  Oj