Hi, this is Bo, an auto expert on RepairPal's Car Genius team!
Well, having only Reverse and First gear in your Trooper is a bit like trying to run a marathon in flip-flops – you can move, but not quite how you'd planned! You're on the right track thinking about sensors or solenoids. Modern automatic transmissions are pretty brainy, relying on a whole team of those little guys to tell them when to shift. If one of the shift solenoids responsible for Drive, or a key sensor that tells the transmission computer what's happening, has decided to go on an unscheduled vacation, then yeah, Drive might just refuse to show up for work.
This could also be the transmission's way of saying "Houston, we have a problem" by going into what's called 'limp mode' – a protective state where it limits gears to get you off the road safely, if not speedily. Since you've already changed the fluid and that didn't do the trick, the very first thing you need is to get the transmission's computer scanned for diagnostic trouble codes. Those codes are like little breadcrumbs that can lead a technician right to the culprit, whether it's a grumpy solenoid, a confused sensor, or something else in the system. Without those codes, we're just playing a guessing game, and that's no fun for your Trooper or your wallet.
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