Do this:

Run Xolox (you do actually use the program, right?)

Click Tools.

Click Preferences.

Select the 'Network' tab.

There's a drop-down menu for upload speed. The reason it's selectable is that xolox can choke your connection when you're doing other things-- so, if you want to use 100% of your upload, select "Max" from the settings drop-down-- NOT 12.

Click apply.

Viola.

I'm sharing about 60GB of "stuff" exclusively with Xolox, and it uses 100% of my 768kbit upload when I want it to... but it lets me crank it down to say, 12Kbytes/second when I'm playing Q3, so my pings don't go through the roof!