Suzuki XL7 2003 Transmission Problem

My Suzuki XL7 2003 (automatic transmission) has started to act very funny all of a sudden. It appears that only the L (low) gear is working and D is defaulting to L as well. Even when I leave it on D, it behaves as if it is on L -- very slow (40Km/h) even when I accelerate with RPM hovering around 9K. The green indicator next to D is not lit up or it is intermittent, indicating an improper gear alignment. However, when I leave it on L, the indicator lights up fine. Even R, P or 2 are not working for both 2WD and 4WD settings. Can someone tell me what could be happening? I took it to the dealership and they tested the computers and everything was fine. They spent some time with it (obviously not the best mechanics in town) and could not come up with anything. Sometimes, this problem goes away for couple of weeks and comes back abruptly. It happened once when I was on the highway and I was lucky enough to pull it to a side and take the following exit.

Is it some sensor? Or, is there some damage to my gear box? Or, is it the transmission? The only thing I did recently was to replace transmission fluid before my trip to Florida (from Toronto).

Thanks a lot for your time.