You really don't want it shifting unless you're using a small motor with not much voltage. When the trans shifts it'll send shock loads through the drivetrain which is t the toughest to begin with. What motor and voltage battery are you planning to use? Also do you have the black or the grey trans? If you have the grey trans you can buy a couple motor pinions in the correct size (need to lookup the size) and make the trans all metal, forward only and single speed.
The black trans can be locked in 2nd fairly easily without the single speed kit. But with the black trans you'll really want to do the double output gear mod that puts an additional gear on the intermediate and output shaft. Really toughens up the trans.