The Drifter by Nick Petrie

Peter Ash came home from the wars in Iraq and Afghanistan with only one souvenir: the buzzing claustrophobia due to PTSD. But when a friend from the Marines commits suicide, Ash helps the man's widow with some home repairs. But he makes an unexpected discovery: a suitcase stuffed with cash and explosives.
'Lots of characters get compared to my own Jack Reacher, but Petrie's Peter ash is the real deal' Lee Child.