Reckon you're gonna end up stripping it down. If grease won't go in theres got to be some sort of obstruction, which is going to be foriegn to the joint, so the best thing is to get it out and you're not going to do that by forcing grease through.
I've just been taking the joint on the back axle of my scammell apart that allows the wheels to oscillate up and down, you would not believe how hard the grease / mud mixture in there was, nothing would have penetrated it.
To my mind its always best in instances like this to strip it down, give all bits a good clean in some decent degreaser and put back together with fresh grease.
Hope you sort it out ok, Richard