Chrysler Repair: Mystery probs after transmission rebuild on 2006 Pacificacid, 2006 chrysler pacifica, chrysler pacifica

Question
I just had my transmission rebuilt on my 2006 Chrysler Pacifica last week.  After I got it back, I noticed a bit of an engine shudder randomly while driving at city speeds (30 - 50).  Now, after driving the car approx 30 miles my car started making a large whining sort of sound and it has started accelerating and decelerating rapidly.  The transmission has been shifting rough from park to D or R, and it is like a runaway train when in reverse.  Takes off with your foot off the gas at least 10 MPH.  The other odd thing is that the speedometer will also bounce from 0 to 10 MPH or so when I am standing still.  Any ideas of what on earth they did to my poor car.  I already took it back for the shuddering engine issue and all they did was add some sort of additive and tell me to drive it 300 miles.  One other thing--I also had them replace them engine mount since that is a big failure point in those Pacificas.  They said the transmission mounts were fine.  I'm not confident that this transmission shop can diagnose this problem.  Any help would be wonderful.    Susan

Answer
Hi Susan,
If these problems were not present before you took it to the transmission shop then I would believe that they have failed in taking out and putting the trans back in to return the engine to its status quo ante. There are many details in such a job (45 steps to remove it, 50 steps to install it) that have to be tended to and something wasn't done right. The one idea I have does relate to the engine mounts in that if the engine were able to move back and forth that could well self-operate the accelerator linkage which might cause the running away in reverse gear. Put the parking brake on, start the engine in Park, then have a helper rev then engine up and down while you observe whether the engine remains in one position or does it rock back and forth quite a bit.
None of those other behaviors suggest a specific cause but let us try to take advantage of the on-board diagnostic capability using the ignition switch to see if there is some aspect of the operation of the engine that has been noticed by the engine computer and noted as a fault code. Turn the key: "on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the odometer window to see the mileage be replaced by 4-digit number(s) preceded by a P. Let me know what those are and we'll go from there.
Please read the PS below.
Thanks,
Roland