lots of people trail ride j-arms. they are just not the best for jumping or hardcore racing.
a buddy i ride with had a nice j-arm shee, it would keep up with my a-arm one easily.
like any banshee, it will need some shocks to make it ride good in the trails. and my guess, the tranny problem is maybe a cracked neutral cover or the screw holding the shifter star isn't seated correctly.