Any loose parts in the trans pan, since you already have it off?
I would be leaning toward a planetary issue like mentioned in post 2 or a forward clutch pack or piston issue since it works in reverse. Forward clutch, the rear position with the input shaft attached to it, is applied in all forward gears but not in reverse.
And also take a good look at the low reverse servo in the rear of the trans case. Maybe post a picture of it as viewed with pan removed.