backlash is a term used in mechanical engineering etc that basically means freeplay!
you need to shim your rear diff on any buggy as close to the drive pinion as you can get it & still have it turning smoothly without binding once the gearbox screws are all tightened up! it csan be a minor pain to get this right & may take a few goes!
the rear diff is the one that is under the most stress in your buggy due to weight transfer during acceleration etc & this is usually the diff that will strip the drive pinion (IF21 Kyosho part#) & possibly even damage the diff ring gear too!
ignore this at your peril, you do NEED to have this shimmed right if you want your buggy to last