Hi
It was a fairly well known problem.
Yes, you can replace the gearbox.
Or, you can fix it.
The tri-lobal plate on early models was a little weaker than needed.
If a glider bumped a kerb, yes the plate could bend a little and give you the result you describe.
To fix it you need a big tool, and a gentle hand.
A big wrench, tire lever, fencing bar, crow bar, names vary in different places.
Lie the Segway on the best wheel.
Observe the high and low points ( a long spirit level may assist ) of the skew wheel.
Gently pry the lowest point up a bit.
Re-measure and repeat.
You may need to nudge it several times to get it close to parallel and straight.
If other wheel is bent also ( which is likely ) then flip it over and repeat.
Then maybe back again to the other side.
You will home in on the solution.
Remember the wheels are plastic ( tough plastic but plastic nevertheless) so use gentle force. I repeat, gentle force.
If this works for you, then please donate $10 to the charity of your choice.
In addition, there is an elastomer between the drive motor and the gearbox input.
These do wear, and can give you a less than ideal ride.
I assume you can still buy them from a dealer.
They should be cheap enough, just a bit messy to install ( remove gear box ).
Michael
www.dgbint.com