Some random told me this last week and all be damned if it hasn't worked every single time since then. If you want a specific piece of raid gear, simply equip a blue rare item after you have killed a boss. Kill the boss with everything you have equipped. After the engrams drop, equip the blue rare item. Pick up the engrams. Every single time since then it's given me whatever gear was in the blue items spot. Got my helmet, 3 primaries, and 3 sets of boots. You obviously can't control the light level but it has given me those items every time. Not saying this is 100%, but the coincidental chance of me getting the exact item I was trying for every time now has to at least increase everyone's chances. Give it a try and report back.