When your slide starts to move, do both sides move at the same time or does one end move a little before the other end starts moving? If it's the latter, you probably have a broken shear bolt. Look for a 1/4" bolt going through the shoulder of the gear. If you only see it on one side of the gear, it has sheared. You can get a replacement shear bolt anyplace they sell lawn maint. equip. parts (snow blowers, rototillers, thatchers, etc.)
It's also possible the pinion gear itself is broken but that's much more rare.