1997 Chevy Blazer trans problem

my trans won't go into the low gears on its own when i put it into drive. but i can shift into them manuel just fine. my brother who is a master ASE Mechanic told me from the code i gave him that the computer spit at me that it's most likely the shift solienoid because i can shift into the gears manuel but it won't go into first on its own and it clunks from time to time into reverse. i can put it into 4 wheel hi but not into 4 wheel low. how do you even replace that part. thanks for taking a read at this guys.