If the fluid is brownish red to brown, then the fluid is burnt. It will smell burnt compared to fresh fluid. If it has been towing, then it is likely that is the situation.

A complete fluid change might work, or it might not. The transmission that year (likely 45 or 46RE) has an overdrive section & a TCC and they are sensitive to old fluid.

The only way to tell if there has been physical damage would be to drop the trans pan & see if there is a lot of metal & clutch particles at the bottom.