Might sound dumb,but have you put the drive and lever in forward,and actually checking both are in forward gear.All the symptoms sounds like thats the issue.Had the same issue putting the drive on a week ago,even by visually looking both drive and gear lever seemed set but they did not,first couldnt get it on and then could not get it off.
Other than that either the studs are bent or the splines on some end are damaged,and you got it stuck by forcing it on.
