You may want to check that someone hasn't swapped out your RD if you have a Shimano 600 on it, the Stumpjumper M2 Pro in 1997 came stock with a XTR RD, which would be an M950.

The 600 is a road groupset, and the RD would be shorter than needed for MTB.

If you have just got the numbers wrong, would be looking initally at the cables.