I can select first gear and ride away no problem, when I select any other gear there is no drive to the rear wheel at all, there is some slight odd noise coming from the clutch side but it seems to be adjusted fine.

Havee you re-engage the tranaxles.; Otherwords taken out the bypass mode?

The are keys ( flat bars ) on the rear axles, if the rear tires have been removed could be the keys have slipped out, with unit in gear check to see if the rear axles are spinning in the wheel hub.

either the belt do not have enough tension or the plastic gears on the drive wheels are worn down or eaten away

If it's a All Wheel Drive then it could be a gear oil leak that has caused a bearing in the drive train to fail. Just adding gear oil won't fix the problem, but may limit the damage until it's repaired.
If it's a Front Wheel Drive, then you may have a Wheel Hub Bearing Assembly failing. This usually has a ABS [ Anti-Lock Brake ] light associated with it.
Either is dangerous and should be checked ASAP.

remove the center caps of the rear wheels then use a 13mm wrench on each side and loosen the bolts for the rear wheels, when loose remove the wheels and axle, then remove the red pin on the fron of the bed, then the two black pins on the rear of the bed, then lift the bed off, then lift the green plastic panel off this will expose the motors then, disconnect the wires from the motor and remove the two screws that hold the gearbox in place then take the gearbox and motor assembly out then remove the two screws that hold the motor to the gear box

The basic explanation of these gears are as follows.
2up/2wd is a basic two wheel drive setting where the vehicle will react as a rear wheel drive. In this gear the transmission will only distribute power to the two rear wheels(best for all around driving with clear road conditions and will give the best fuel efficiency).
4up/4wd is a high speed four wheel drive setting. in this gear the transmission will always distribute power to all four wheels and will allow you to drive at higher rates of speed( good for poor road conditions when ice and wet roads are a moderate risk).
4dwn/4lo is a low gear four wheel drive setting where again the transmission will distribute power to all four wheels but will limit you to a low rate of speed(good for really rough roads and severe weather or very poor traction conditions).
auto gear selection is basically a all wheel drive setting, where the transmission will use a primary setting of rear wheel drive gearing until the vehicle slips or looses traction on one of the drive traction. as soon as the transmission detects a difference in wheel speed between the two rear wheels, at which time the transmission locks up the front differential turning itself into a four wheel drive, until the wheels have regained traction and remain spinning at the same speed for a set amount of time, at which time it will then unlock the front differential at return to a rear wheel drive setting. this is a good all around gear but will cause you to get slightly less mpg/kpg then a 2up/2wd gear would. i hope this helps

some times you can add a thin shim to the out side of the wheel mounting center to push the wheel gentally a little more to the drive gear for more positive engagement to the wheels inside gearing, also check your gear drive mounting pins/ keys for wear if thier warn it will engage the end of the drive shaft iradically appearing as slipping type movment at the wheel hope this Helps Steve

Before stripping the gearbox be sure it isnt just the gearshift pedal rotating slightly on its shaft. The splines on the shaft will ruin the pedal if its not tightened correctly.
Otherwise it could be the gear selector mechanism. The spring on the shift claw or detent arm may be broken.
Check it out by rotating the rear wheel whilst the bike is turned off & on its main stand, then select each gear by hand with the pedal.

Good Luck
Hope this helps.
(If it does then please mark this as 'useful')
Ride Safe
Bike-Doc

Its possible that the cable slipped does the pulley turn on top of trans.if so does the shaft turn if so then your gears on wheels need cleaned or replaced.

As far as I know the procedure should go something like this...

Stop the vehicle, engage the parking brake and if applicable, get out and turn the hubs on the front wheels from Free, to Lock.(You may not have these on a Suzuki, they may be auto-lockers)

Get back in the vehicle and put the transmission shift selector into Neutral.

If you have a manual transfer case, move the selector-shaft to 4x4-High, or 4x4-Low -- whichever is desired. Now, putting the vehicle in gear with the transfer case is sometimes somewhat difficult. As long as the vehicle's transmission is in neutral, don't worry, the gears inside the transfer case are big and beefy -- Don't be afraid to be rough -- Again as long as the transmission is in neutral. You may also have to disengage the parking brake and slightly let the vehicle roll forward or back with your foot-brake to get the Transfer case to lock into place.

Once 4x4 of the selected High, Or Low range is selected, usually indicated by the vehicle jerking slightly with a slightly loud clank/thud. Move the Transmission selector out of neutral into your desired gear (Drive if Automatic, if Manual, whichever gear you pull out from). and voila! You should now be in 4-Wheel-Drive.

To disengage 4-Wheel-Drive. Just reverse the steps.