Give my $.02
Those JumboMax XL are huge & I'm willing to bet it's thrown your timing off in regards to closing the face, especially your right hand[rh golfer]
I tried these exact grips....had success even- but ultimately ran into your very problem - not the shanks but damn close.
I had to "throw" my right hand at the ball, to have success.....its ultimately lead to me rebuilding my grip & grip at impact.
Right hand on top[at impact] is a major hidden swing flaw - and a mother to fix apparently
I don't use JumboMax anymore