I agree, I think you did damage to the splines on the differential, or possible even broke the spider gears. I've seen the pin that goes through the spider gears get worn out really bad before, it's possible that it is what is causing your problem. When that gets worn it allows the spider gears to move up and down. Eventually if it gets bad enough something is going to snap in there. Not saying that is your problem, but it's a possibility. That would be a horrible coincidence for that to happen right after you just replaced the clutch. It really sounds like it's time to have a shop take a look at it and get an opinion from a shop.