First off, change the tranny fluid and filter. Also check the cable from the shifter to the tranny. If its gotten loose it may not be fully engaging the tranny into drive. With the car off pop it into drive. On the top of the tranny near the back there is a lever with a cable attached to it. this is what selects the gears inside the tranny. You should be able to pop it in between drive and neutral. Mark the position of the lever with white out when its in nuetral. Move the lever with your hand for this. Now push it into drive with your hand and make another mark. Hop in the car put it in park and then back to drive with the shifter. Now look at the lever on the tranny. If the marks arent lined up perfectly the shift cable is loose.
The jerkyness around 45 mph is probably a sticky EGR. And the eratic idle is most likely a bad IAC.