Leon tells Toni that the Forellis have a gun-running operation, and are tooling up for trouble. The pair sees it in the best interests of the Leone name to put an end to the operations.
There are two trucks that are carrying an order for the gun-run. They both have the same routes - beginning in Torrington, and finishing at a lock-up in Fort Staunton. You must make sure not one of these orders get back to the Forellis. It won't be straightforward than the missions that have gone before, though.
If you happen to acquire a Rocket Launcher (or just buy some ammunition from Phil Cassidy's) then the whole thing has a new perspective. If you go about this normally, you'd either get capped by the guards in the truck, or by the reinforcements they call. With the RPG, you'll just have to let off one blast on each of 'em.
Whichever method you use, costly or not, you'll need to know where these trucks move. One of them will have a no nonsense driver that'll go through the main roads to the lock-up, and the other will take the loop around the Big Shot Casino. If you are using the RPG, test and adjust your vantage points until you get great shots on the both of them. Luck should be on your side with the drive-by technique to destroy both trucks and complete the mission.
Leon will call after this mission, informing you that the Feds are catching up with Salvatore, and that he's about to be framed for every gang-related crime that's been pulled off since Toni's comeback. This will end Leon's set of missions.