trap bar dl focuses more on the quads for the lift, while standard dl focuses on posterior chain. dont drop standard dl from the routine, lighten the load and focus on speed work.
what other assistance work do u do for dl? reverse hypers and goodmornings will do wonders for you. chances are the back pain you are feeling is from the psoas. you need to do alot of stretch work to avoid injury as well. i could go on and on...since dl is my specialty.