Gaffney, 26, had spent the last two seasons with the New England Patriots before latching on with the Jaguars. He's yet to take a regular-season snap since he was selected in the sixth round of the 2014 draft.
Grimes, meanwhile, spent the last four seasons as a seldom-used rotational piece in the Houston Texans backfield.
During that span, the undrafted free agent out of William & Mary managed 139 carries for 613 yards and two touchdowns. He also flashed some promise as a receiver to the tune of 51 catches for 429 yards and a score.
The Jaguars have a loaded depth chart at running back with rookie Leonard Fournette, Chris Ivory and T.J. Yeldon all in tow, so Grimes figures to be fighting an uphill battle for a roster spot.
Also consider Grimes has been left with just a few weeks to learn a new system, and his best hope may be to put solid work on tape for other clubs that could use more reinforcements at running back come September.