Would a pistol grip decrease muzzle jump ?
Indirectly yes. It gives you much more control of the gun and allows the hand to absorb some of the recoil. What some see as muzzle jump is actually loosing hold of the gun with standard grip stock. A pistol grip will fix that.
will i need to resight?
I don't see why you would need to resight. I you are using straight bead sights, then the drop in the new stock may affect POI/POA, but if it has an optic everything should be the same.
I use a pistol grip stock on my 835 with factory beads and have had no problems. If you go with a tactical type stock then you will probably need a raised sight.