The substance that you are seeing out the tailpipe is coolant mixed with the normal exhaust coking of the burnt diesel fuel! The leak in the egr cooler is gradually getting worse and using more and more coolant! The reason that you see more smoke on deceleration is probably due to the elevated engine temperature and engine speed under load, where it is evaporated before it exits the tailpipe! Once you coast, you do not have as much heat to burn off the coolant and it will flow out of the tail pipe as steam mixed with normal exhaust fumes! Below is the ford technical service bulletin that would be followed by a ford technician at the dealer to correctly diagnose and repair this concern! I have done enough of these to know that this is exactly what is happening to your truck!
An aftermarket cooler will most likely not be more durable than the factory one, this is due to the fact that it is the oil cooler that is restricted that is causing the EGR cooler to overheat and fail!
DOWNLOAD TECHNICAL SERVICE BULLETIN HERE
Yes, the most critical thing that you can do to save your truck from having this happen again after you replace the oil and egr coolers is to flush the cooling system as described in the technical service bulletin! Then when you are finished with the flush and have drained the block of all tap water and iron flush, is to fill the engine with motorcraft premium gold anti freeze, and use an equal amount of distilled water and not tap water! Then you will want to drain and fill your cooling system every two years or less with the same coolant and distilled water mixture!