you didn`t say if it was fwd?rwd? on fwd there is a needle roller bearing on top of the selector mechanism,you need to take the 6 10mm bolts off holding it onto the box,you will then see the shaft which goes down through an oil seal and through the two needle rollers,the top one is bad for seizing and drying out causing sticky selectors,not saying this is your fault but worth a look anyway if the motor is fwd.
wickham
August 2010