I had this same problem about six months ago. At that time I injected a new NIC drive into our boot image so our TS would work. We just received a brand new e7450 and it seems Dell has change the driver set yet again.
I need to remove the existing NIC driver that was injected so I can replace it with a new driver, but can't remember how. Here's an article I'm trying to use. (using SCCM method 2)