To answer your question - since it's not the government doing anything, the ACLU is not really interested. What exactly COULD they do? They cannot exactly SUE Sheehan to remove them, because it's not their land. Do you see my point? The ACLU really couldn't do anything if they wanted (as far as I can tell).
Now, if the government (say local county or state) tried to PROHIBIT the Sheehan kook from putting crosses on public property (i.e. along the road) then it would be very interesting indeed what the ACLU would do. THAT would be fascinating !!!
Would they sue on behalf of Sheehan, in order to protect her 1st amendement right - and creating a precedent that would allow religious people to put crosses on public property anywhere they wanted to ?
It's also interesting because there is already a precedent for "allowing" crosses by the side of the road - where loved ones place them after a traffic accident. I'm sure it's not a legal precedent, but if someone tried to outlaw what Sheehan was doing, then that might prohibit grieving family members from putting up crosses.
I guess it's all a moot point anyway - since she'll be on private property anyway soon.