louden_swain: there are different opinions of what a strong core is.
You shouldnt be feeling this in your upper body AT ALL.
With out watching you do the lift, your elbows may not be high enough throwing the weight forward....or gulp you have a weak core (you your self just said you feel it in your lower back). It is a priceless lift if you can master it...but it isnt essential (as i said before).
If you are interested.....try brining up your good mornings....that will help
jeremys: tight triceps? I had a lot of trouble until i stretched my way into them. Lats, forearms, quads triceps, seemed to hold me back the most
If you watch some of the superheavy OLs some of them hold the same arm position as you while recovering from a clean...