That's not correct. A GM TH400 transmission should *not* drive forward in reverse or neutral. If your TH400 is driving forward in reverse or neutral, there's a serious mechanical problem within the transmission. This is likely a significant internal failure, possibly involving the planetary gearset or a broken linkage. Driving it in this condition will likely cause extensive and costly damage.
You should immediately stop driving the vehicle and have it towed to a qualified transmission specialist for diagnosis and repair. Continuing to operate the transmission in this faulty state will likely lead to complete transmission failure.
