Rear shock fourtrax 300 4x4 1993

Hi i want know what is the difference between a rear shock of a 4x4 trx 300 and 2x4 trx 300??For me the rear of the 2x4 is the same rear of the 4x4??I dont understand why some online seller sell rear shock for the 2x4 trx300 and say that the shock will not fit on the 4x4.Why???