THIS ANSWER EXPLOITS A GLITCH IN THE GAME
bit of a late answer but its been bugging me that there isn't one, so if you don't mind using the duplication glitch/grinder all you've got to do is find one shield of the correct level from a vendor (took me about a half hour of flitting between elseer and Concordia)
host: start game in Concordia.
p2: sign in.
host: drop shield/any item you would like doubled.
p2:pick items up and walk past "new u station" to save, drop everything you would like and immediately sign out via home button (this does not give the game a chance to save so when you sign back in the weapons/items/shields that you had originally saved are still there plus what ever you had dropped).
host: pick up items - save.
p2: sign back in - repeat.
once all the shields you want have been accumulated grind three legendary shields together at the cost of no moonstones :D (sometimes you can get an exact copy of the three shields you have just ground, some times just the same type this happened to me first few grinds I had done, so dupe plenty of shields and persevere I've gotten an 81% sham from three identical whiskey tango foxtrot shields).
note!!! occasionally the game will kick you out because of a "sign in change" I believe gearbox has done this to circumvent this glitch from bl2 but it is still very possible provided you use the same save stations (I use the one behind the jump pad near the grinder) and only save when your host is the only one signed in, unless of course you loading your mule full of shields, then you need to save their inventory just the once so you don't lose all your shields.
I know its a very convoluted way of doing things but its the only way I could figure it out as they've nerfed all the drops, you cant farm any bosses apart from Irwajira,
Nel and the Sentinel and also you can apply the same logic to grinding with moonstones( eg purple orange orange) as long as you use p2: to grind and drop the legendary, sign out, repeat you'll be fine
I hope this helps