extended shackles will give more flex
can you not see what is stopping it flexing further when its at full droop? i would say the shackle is hitting the box that it sits in and maybe the shocks run out of travel. if you have lockers front and rear you dont need flex to be honest.
These will help http://www.ironrockoffroad.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=IROR&Product_Code=RC-1117&Category_Code=PFX