So I have had this car 5 years now it's only worked one time and never worked again. I'm looking for either help figuring it out or some sort of diagnosis tree for doing so.

Info:
I have checked to make sure it has power and ground going into the pan and that I can hear the solenoid click when the button is depressed which I can. The transmission was rebuilt 3 years ago because it ran low on fluid and trashed the clutches. The whole clutch pack steels and all were replaced. Car currently drives fine, shifts through all the gears and hits OD at 45mph. The only weird thing it does is when it's put into reverse it takes a second to actually start to roll itself back. If you push the gas it'll go back immediately. I talked to some one today on the phone who wants 900 bucks to go through it and replace a sprag. I talked to two of my transmission buddies who have both worked on gm transmission for a long time if that could be it. They both bluntly asked me if the car moved and said if it did it's not that.

Any help here would be great. I don't work on transmissions at work nor could I tear one down but if I can narrow down what it is I know two get reasonable people who could replace a bad part. Thank you in advance.