If the older tires only have about 1/3 tread left I wouldn't want to risk it myself. Tthat makes a big difference in circumference compared to the new one. 33.3% tread vs. 100% If you were going to run them I would put the new tire on the front since you have an open diff up there so it won't burn out the LSD.

Mike
2005 Toyota 4Runner SR5. 1994 Isuzu Amigo XS 4x4
http://community.webshots.com/user/98amigo