Okay, this is kind of embarrassing; I fix bikes as a small summer business kind of thing, and I just got hired to tune a specialized crosstrail mountain bike. It looked easy enough - straighten out the brakes, clean, new grease - all in all a 20 dollar job maximum. Then I got home and realized that the bike did not shift higher than 4th gear (as in it can shift to gears 4,5,6,7,8 but not 1,2, or 3). My first reaction was to check the rear derailleur because I figured that the tuning screws were blocking the derailleur prematurely. That was totally discredited when I disconnected the cable completely and the problem persisted. So it's something in the shifter for sure... and after opening it up I decided that taking it completely apart was a little beyond my comfort zone. The shifters are those standard mountain bike shifters (the clicky ones, not twisty) by shimano. Additionally, when I opened up the shifter housing, I observed that the shifter mechanism goes through its entire range of motion (from one metal stop to the next) as it shifts from gear 4 through gear 8, almost as if the shifter mechanism is really a 5 speed that somehow got put into an 8 speed housing. The left hand shifter has a similar problem; it feels like a 2 speed shifter mechanism housed in a 3 speed housing.
Any ideas would be a great help. Also, if this explanation didn't make any sense, please tell me so cause it was sort of a strange thing to explain.