P2002 indicates the exhaust particulate filter is operating "below threshold" or excessively plugged. 

You will want to go to the dealership for this concern for a couple of reasons:

1.  The dealership diagnostic equipment has the ability to command your truck to perform a manual regeneration of the exhaust filter.

2.  If the manual regeneration fails, there is a good chance replacement of the DPF (Diesel particulate filter) is covered by the emissions warranty depending on the mileage of your truck.

Other things that can cause this include but are not limited to:

Dirty air cleaner, modified tailpipe, aftermarket performance modifications.  Do you have any of these conditions?