If i rent a truck do i necessarily have to use it for menards tools or can it be like picking up something else from a different store??

You would be best advised if you stated the truck's usage. Your usage might be a liability issue if Menard's insurance coverage is used. It's always best to be safe than sorry.

You can use it to pick up items anywhere. It's your money

I don't think you have to tell them what it's (actually) for.

You can use it for what ever, hauling, removing stuff etc.