Q: Transmission problems?

While sitting at a complete stop yesterday, brake pedal fully depressed, my truck started a high idle, then started to shake like it does if you let off the brake slowly. I know my rear shoes need to be replaced, but the issue i seemed to be having did not appear to be brake related, as my foot did not move. However i did try to press the brake pedal harder just to check and nothing changed. I put the truck in reverse and backed up and it stopped when i held the brake pedal again (with the truck in drive as it was before). This may possibly be related to prior transmission problems that were never addressed because it stopped when the ignition control module was replaced.

(Little bit of background on that problem just in case: the truck would idle fine, but occassionally stall out when put in reverse. Especially when wet. Ended up having to start the truck and immediately put it in reverse before the idle could die out. Changed ignition control module and this problem has stopped.)

My car has 244000 miles.
My car has an automatic transmission.