Drive shaft moves in park!

[Ohtooman]

My drive shaft moves back and forth when the car is idling in park.
I have a horrible suspicion that it is transmission related. Has anyone seen this before? Car has original tranny, 335,000 miles

Thanks

[leathermang]

We need a better description of this to help you .
Moves forward and backward ?
turns clockwise and counterclockwise while just sitting there?
How can you see this happening ?

[Ohtooman]

When I pull up and stop, I can hear something thumping. I had assumed it was my exhaust, but after looking under the car with the engine running, I can see the driveshaft rotate slightly, then snap back. It oscillates back and forth in the normal rotation direction. It doesn't move very far, maybe an inch or less, then rotates back. It is as if power is still coming from the tranny which is in park.

[leathermang]

I don't know why power seems to be being applied to the driveshaft while it is in park....
but the noise you hear may not be in the trans.... but may be play in your ujoint.... which would be better than in the trans...
When did you change your fluid and filter last ? I would try doing a Trans-x treatment ( do search for instructions).... because most things in the trans are controlled by hydraulics... and the controls for the hydraulics are small holes which can be clogged by varnish over the years....
It would appear that a clutch is not fully disengaging...Have you checked the manual to see if there are any outside the case adjustments available on your transmission ? This would usually look like a bolt with a lock nut around it.. which would be loosened ... the bolt backed off part of a turn and then torqued to a specified reading.. then the lock nut tightened.... you may or may not have this type adjustment available... but this is the first place I would look on an old American car if the power were still being tranmitted in park...
You may have a broken " parking pall"....

[Ohtooman]

Transmission oil and filter were changed about 10K ago. I may in fact have a bad flex disc, but surely it wouldn't make the driveshaft turn when the car is stopped?

I'll try the TransX, I'm sure it wouldn't hurt. Another thing that the
tranny does is this: First thing in the morning it will always back up just fine but when you put it in drive, it will hesitate like it is out of gear. When the revs come up, it will lurch into gear and go.
It never does this the rest of the day.

Thanks for the suggestions.

[leathermang]

When things will work... but are slow starting to do it is one of the most common things for Trans-x to fix.... those changes are causes by hydraulic pistons and thus are directly related to pressure and FLOW of fluid... if varnish is stopping up a control valve hole then the things work slower than they should... in the worst case it slows the actuation of the clamping ability on the clutches... and allows slipping and excess wear fast... like riding the clutch....
Be sure to find out how much fluid your trans uses...and read the instructions on the Trans-x... it may take two cans to properly dissolve the stuff... my wagon did..... If you do a search you will see lots of my instructions on this ....