Se height e' pari al 100% le barre laterali non compariranno mai...

.divcentro{
height:200px;
width:100%;
text-align:justify;
SCROLLBAR-FACE-COLOR: #6699CC;
SCROLLBAR-HIGHLIGHT-COLOR: #698BB9;
SCROLLBAR-SHADOW-COLOR: #A8C5E1;
SCROLLBAR-3DLIGHT-COLOR: #A8C5E1;
SCROLLBAR-ARROW-COLOR: #F4D900;
SCROLLBAR-TRACK-COLOR: #6699CC;
SCROLLBAR-DARKSHADOW-COLOR: #6699CC;
overflow:auto;
}

poi usa <div class="divcentro"> e non span