Yes, if we don't plan the organisation of content properly, it will be a bad experience for the user to have this sortable (drag n drop) thing within a scrollable content. Again, on the positive note this drag and drop is one of the recognisable feature on Mobile for users, where they see and use this feature in few apps which they will be using in daily life (EG: Notes app in iPhone)
This drag & drop option will work better if the page is of fixed height.
May be re-looking in to the page content organisation, so that the page stays in a fixed height and user can use this sortable (drag n drop) option with out effecting the whole experience.
If there is no chance to re-organise the content as said here in (Option:1), you can still list these 5 items within the lengthy scrollable page, and can give a button "Sort my items" near to those items. By clicking on the button, we can show this sortable (drag n drop) option in a overlay, which should be specific to sort these items.
As these items are being shown in overlay, user can easily sort these items without interacting the lengthy scrollable page (which is in background) as it will be locked.
This option may not be user friendly as it requires more number of clicks and more of error prone. This is the worst case option, i'm just trying to throw an idea.
If Option1 & Option2 doesn't work, we can still show these 5 items within the lengthy scrollable page, and can give a button "Sort items" near to those items. Once they clicked on the button, we can then show the (counter/drop down) with the numbers 1-5 for each item.
So, users can select a unique number from each 5 drop downs and then click on the button to sort them.