1. deploy shield
2. sprint (so your character put his shield away briefly)
3. whilst sprinting hold RB or LB to deploy a throw able (i used Medipack)
4. either toss the item or press b to cancel
5. you character will get his shield out and you can switch weapon
Pull out your shield. Have a turret or medic bag in the other skill slot. Sprint and while sprinting hold the turret/med skill. Simply press circle, and then triangle to switch through your weapons. If you sprint again or do anything that puts your shield away temporarily you will need to do the glitch again. You get use to it very quickly.
Edit: 03/23/2016 Wanted to mention this is patched. You can change weapons still and shoot, but you will do no damage with your primary or secondary.