TH350 itself into 2nd/1st?

[mchyphy]

I have a problem that whenever I take off from a stop with a modest amount of throttle, the shift lever will move itself into 2nd, 1st if I really gas it. It started happening after I changed my motor mounts because they were bad, and the new mounts seem fine. It was suggested to me that the transmission mount was bad and might've shifted allowing the trans to move and cause it to shift itself. Thing is, I just replaced the trans mount, which was in fact shot, and it's still doing it. I'm not sure what else could be causing this. Any help and advice is appreciated.

[Gromit]

This is going to be a little superficial because I don't have time to pull the proper pages from the service manual at the moment;

But because no one else has spoken up yet I can offer this to try and help you with your situation until the big dogs that I know will be reading this chime in:

There needs to be some play fore and aft for the driveshaft assembly between the rear pinion and the slip yoke where the driveshaft meets the transmission.

You can google this to look at the slip yoke part: Dana Spicer 2-3-4911X

Because as you probably know when the rear suspension moves up and down the drive assembly needs to expand and contract to make up the difference.

If there is no play then that could be a possible cause of the condition you describe.

Assuming there is too little play the fix would be to loosen your motor mount bolts and trans mount and shift the whole motor and trans assembly forward toward the front bumper about 1/2 inch until there is about 3/4 of an inch (this is just as I recall - I don't know the true spec) of fore and aft play available in the driveshaft and slip yoke.

The aftermarket sells a dual slip yoke so that you have a second one where the driveshaft meets the rear differential; but as they came stock all of the play was supposed to come just at the transmission slip yoke - so in other words a single slip yoke as opposed to a dual slip yoke.

Hope that makes sense; you can look at the Denny's Driveshaft website if it is still around as they had some good information.

Sorry that is all I have time to offer at the moment.