4L80E won't back up

Well here we go..sort of if i could back my truck up! We just rebuilt my 5.7 in my 96 GMC sierra and I mean just rebuilt! Drove it home and went to back it out of the drive way and it won't go. When I put it in reverse you can feel it engage but it just won't go!!! So if anyone has any Ideas thoughts or dinamite let me know cuz I am getting fed up.

This is internal it is coming out and apart, usually a bad sun shell causes this.

agree with max