Transmission feels like it slips sometimes. Also, the auto door locks go off all the time and door open light and sound happens even when doors are closed. What can I do?

Well the door thing and trans thing are NOT related.
The door issue is because the GEM thinks the driver or passenger side doors are open, it's sometimes caused by the door switch in the latch assembly inside the door itself , it makes it easier to have a scanner (I.e. Modis or solus ) to see what the doors are doing to avoid wasting $$ on needless repairs.

You should be able to have a transmission service done to replace the transmission fluid and filter & see if it improves the shift quality. Also make sure you use the Proper type of transmission fluid, most ford's take Mercon V type fluid.

Your door ajar concern is caused by one of the door ajar sensors showing open, The sensors are located inside the doors attached to the bottom of the door latch. you must remove the door panel and latch to access then to clean and re-lube them. A ford tool will tell you which door it is. Without this you must do this repair for each door.

As for the transaxle odds are its above 40mph when your feeling this, which is a torque converter shudder which can be resolved by new mercon 5 fluid and additives like lubegard or smartblend or Dr shudder.