It will be a mess! Just to warn you.
First you have to change the top-position of you div so it start lower than the menu.
Second you will need to assign a negative z-index to the div so it will stay under the menu.
And then the trickky part comes:
To hide the text that should not appear until you scroll, you will need another div that starts just below the wanted bottom and all the way down to the bottom of the viewport.
And you will need a bottom-padding on the text-div of the same height, so it is possible to scroll down to the last line of text.
It can really be a mess.
A better solution might be to make the scrollbar appear as far to the right as possible.
<div id="scrollDiv" style="position:absolute;top:250px;left:150px;right:0px;height:350px;width:auto;overflow:auto;overflow-x:hidden;">
<div id="textDiv" style="position:relative;top:0px;left:0px;width:600px;margin:auto;">
Insert Content Here