Backlash can come from different things. The meshing too lose, the gearbox, the coupling or the end blocks. If you have a spring loaded worm then you can rule out meshing. Gearbox backlash is usually very minimal. Check your couplings that they are tight. The end blocks is the most likely. If the gear moves axially between them you need to unsecrew one, squeeze the worm between the two and secure it again.
