The forums look fine to me. I don't see any scrolling even when I make the window much smaller.
edit: Nevermind, I see you said in the threads.
Your scroll problem is because of this guy -
http://www.truckmountforums.com/forums/26990-post6.html All the info is stretching out the post width. The td tags in the table for the user portion of the postbit are all set to nowrap. Get rid of that and the words will wrap and stop pushing the thread width out.