I would try slackening the tension right off on the left hand track and see if that makes any difference. If the tracks have been rebuilt with new rubbers etc it may be that you have what was the right hand track on the left hand side which may be running tight on worn drive wheels. If you take it for a long drive around a field it may settle in ,if not try swapping the tracks. another thought is that you may have the track reversed from original which would mean the left track is driving on a less worn side of the track driving teeth try removing the left track and rotate it. Clearly something on the left side is running tight causing the diff. to power the left track. If you do end up removing both tracks when you have the machine securely jacked and propped, try starting the engine and engaging gear and check that both drive wheels rotate together, this will eliminate a fault in the hub/final drive bearings.
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