97 Subaru Legacy it jerks on few turns during AWD but it doesnt on FWD.?

Internal binding of the AWD system. Common problem for these cars. Usually happens when the trans oil does not get changed regularly. FWD mode turns off the solenoid that controls the rear portion of the AWD system. If it is in the early stages of shuddering, perform a flush (16 qt flush not 4 qt drain and fill) add some friction modifier - the GM stuff works well. Try to use Subaru trans fluid. Dex III can be substituted but the Subaru stuff is superior. Drive the car in clockwise and anti clockwise tight radius circles in any car park to free up the binding plates. Hopefully, the rear drum that houses the AWD clutches doesn't have notches embedded into it (will require a rebuild at that point) because it will be permanently locked at that point - the only way to be certain is to disassemble the transmission or at least the rear portion of the transmission. A skilled technician can rebuild that portion in car. In some cases the transfer solenoid is bad but since you can turn FWD on, that is unlikely. The differential built into the rear of this transmission. The person who rebuilds this transmission if it comes to that must be familiar with this car. This transmission is built to aircraft specifications and the tolerances are tight. This is a job that should not be rushed.