Figured it out: I used FullModeItemTemplate instead of the regular ItemTemplate in my xaml.
<toolkit:ListPicker x:Name="friendPicker" SelectionMode="Multiple" Margin="129,283,10,119" BorderBrush="#FF8DCDC1" IsEnabled="False" >
<toolkit:ListPicker.FullModeItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Name}"/>
</DataTemplate>
</toolkit:ListPicker.FullModeItemTemplate>
</toolkit:ListPicker>