Most often its a shifter linkage adjustment. Also, as CFM rubber-mounts it's engines on rubber mounts, the more one shifts between reverse and a forward position, as you would plowing snow, the more those rubber mounts get torque-slammed and the quicker the wear out. When they get too loose they tend to pull on the shifter rod and as the shifter is locked in a position with it's gate, the more often it will be pulled..or pushed.. out of the transmission's detent. Better check them too. Doing this also tends to bend..then break.. the shifter fork in the transmission if it happens under too much load. Let's hope that hasn't happened.