Jodi,
I dry large slabs like your walnut here in Western Oregon, so have some experience. I would agree with Tenn. Jim that if the bark is loose it should be removed before drying. In general, trees that are cut down in the spring and summer retain the bark more tightly than fall and winter cut trees. You will not cause any checking by removing the bark and it will reduce insect activity.
I also would caution about placing the slabs into a barn. It is important that the slabs get plenty of airflow through the stack, but if the air is fairly dry the wood may dry too quickly, causing defects. Here in Western Oregon the winters are extremely wet so the relative humidity stays high, while the summers dry. I try to have most of my sawing and initial air drying occur during the wet winter months which favors very slow (if any) drying. Then, during the late winter and transitioning into spring the outside air slowly starts to dry out which provides a gentle drying rate for the wood. The most critical part of the drying process is from green to around 20% wood moisture content, so this initial slow drying is critical to success especially when drying thick slabs. By mid to late summer, the wood gives up the most moisture to the dry air and then in the fall the drying rate slows again with the rains. If you are in other parts of the country the wet and dry seasons are reversed (cold dry winters and humid hot summers) and the rates of drying (which are a function of the relative humidity in the air) may be different. So, if you place the slabs into a barn, place the stack sideways in a breezeway or near an open door to get reasonable airflow through the stack. Do not cover the stack with a tarp as this will disrupt the airflow and cause mold and poor drying.
I let 3" and thicker walnut and oak air dry on stickers for at least 4-5 summers before going into the kiln. Special wood requires time to dry properly, so be patient.
If you do a search of this forum on the topic of drying walnut slabs you will find more on how to dry and a few photos of finished works from slabs (including a few of mine).
Best of luck.