What about getting some of that really sticky black window cement and glue em back down? I'd imagine you could drop some welding rod or some sorta wire in the channels to keep em from sliding too.

JK

That's a good point. If you arent wanting to spend any money and don't need the sliding feature, you could shut it permanently, which would be more secure than even a working slider window, which is one of the common ways through which these trucks get broken into.