Excel making a cell available based on the criteria on 2 columns

I have a list of spare trucks. I have column A with trucks returned and Column B with loan trucks that have been given. I would like to not have a truck selectable from a drop down list unless it has been returned (Until it is put in again in column A) Now since the trucks are going to be in and out multiple times I need the latest entry to take effect. For example I might have truck number on Column A (dropped off truck) which is going to be available until there is an entry on Column B(Which is picked up by someone) but a few weeks down the track the truck will be returned again (Column A) and needs be available again. Is this doable or am I trying the impossible?