First of all, welcome to the forum, and congratulations on your purchase of the Venture Lite! I hope it will be a good sled for you once this is all worked out.
Regarding the shimming that you keep hearing about, it's probably in reference to shimming the secondary clutch a bit to keep the sheaves off the belt at idle. This way, the secondary/jackshaft doesn't have any tension on it from the belt and will be free to move a small amount to allow the servo/gearmotor to shift between gears. From what you described, I am thinking that this is not the nature of your problem. I wish there were an easier way to diagnose this, but I think motojunkie is right, it's going to take some testing.
On the reverse gearbox, there is a sensor/switch on each side that give a signal to confirm that the shift has taken place and the gears are fully engaged. I'm not sure how it would happen, but I would imagine that if somehow the gearmotor got stopped somewhere between the two, it wouldn't know where to go and would just sit there, with no forward or reverse light lit up on the display. I would think it would also be physically stuck between gears, though it may be possible to have enough engagement on forward (or reverse) that the sled will move forward under power just fine, but not enough engagement to trip the sensor/switch and turn on the 'forward' light on the display.
I've attached a couple of pictures showing the location of the forward and reverse switches, though they're not what I would call easily accessible. One is somewhat hidden under the corner of the chaincase cover and the other is on the 'inside' of the sled, near the exhaust, most easily accessible with the fuel tank removed. If you need another excuse to open up the sled this far, checking/changing the spark plugs as well as the exhaust 'donuts' can also only be done with the fuel tank removed. You will also have good access to the main power connection for the gearmotor once the fuel tank is removed. For the other switch, you could get to it a little easier with the chaincase cover removed, which would be a good time to replace the gear oil and check the tension.
I've also attached a .PDF file which includes a schematic and some troubleshooting techniques. It also looks like there are 3 separate relays used by the reverse gearmotor during the shifting process, so it may be easiest to test those first in case they are at fault. That way you won't have to tear into the sled as much. The relays should be right near the battery if I remember correctly. Good luck, and I hope this helps!
