That happens a lot when using odd transmission, driveshaft combos. I've heard of it when a TKO trans is put in a SN95 car. The problem is that trans is shorter that the t-45 not allowing full engagement of the slip yoke. Damages not only the slip yoke, but also tends to twist the output shaft. This makes it very difficult to remove the broken slip yoke. If you've done that kind of damage you need to do a few things before proceeding.
Check the length of the drive shft and make sure it's not a bastardized unit. It may be too short and could have caused the problem. Pull the trans for inspection. Once you do get the yoke out check the splines to see if the sheft is twisted or possibly BENT now. If it's just bent a little it's junk. Replace the shaft.
Good luck!