1999 Chevy Camaro shifting problem

1999 Chevy Camaro mileage: 60,000. The transmission shifts into reverse fine, and it also shifts from first to second fine, but when it shifts into drive it reacts like it was shifted into neutral. Are there sensors or something like that causing this problem? In addition, if so what are they and is there a way to check them to see if they are bad short of taking it to the shop? I changed the fluid and put a new filter and gasket in transmission and there was no difference. It has been driven very little since problem occurred. Any help would be greatly appreciated