Re: Transmission fault codes (?)
The P1784 code means LR pressure switch circuit. The other two codes mean that the transmission is slipping in 3rd and 4th gear.
After ignition off, does it shift normally? Most of the time, when it goes into limp mode, it will reset after ignition off. If the fault is still there, it will go into limp mode again.
Fluid level is normal, but how it the color and smell? No burnt smell to the fluid?
I would start by checking the large connector on the solenoid pack. Check for moisture or corrosion on the pins.
If that looks ok, than you most probably will need to pull the pan and check for any debris, but first check the following:
It can be that the line pressure is too low, causing the slipping in 3rd and 4th gear and the failure of the LR pressure switch. You could try to remove the line pressure sensor connector on the RH side of the transmission housing and test again. The TCM will set the pressure regulator solenoid in a fixed position in that case. If it than works ok, i would change the line pressure sensor.
If still not ok, than you will have to remove the valve body and check the clutches with air pressure and listen for a large air leak inside the transmission. If no large leak is present, you will hear a thumb from the clutch piston when it actuates. If no leaks present internally, than you can have a problem with the solenoid block (again), or the gasket between the solenoid block and the valve body. This gasket is quite fragil and can normally only be used once.