Vibrating Van HELP! 88 2wd

I had a small clunk when decelerating as I almost stopped and it has developed into a serious vibration (Or I have a new problem). When i accerate to 60-65 mph I get a terrible shaking that continues to shake untill I am almost stopped. when I get down to 10-15 mph it smothes out and doesn't come back untill I get to 60-65 again. I feels like it's from the driveshaft area , but that checks out ok. I shakes so bad the dash bounces. I've checked out the whole front end, u-joints, brakes mounts..... It drives like it's new up to that speed then starts in again. I'm puzzled by it continuing to shake untill I'm almost stopped and then smothing out. the u-joints are non greasers. Could one be dried out? I'm thinking torque converter, but I don't know. Any help would be awesome. Thanks in advance.

I'd lean towards a u-joint locking up. If they are non-greaseable, they are probably the originals. If you find one bad, replace them both.

Thanks for the first quick reply. I'm gonna change them on monday so I'll post if it was that. Any other thoughts would be helpful in case it turns out not to be the problem. Oh yeah.,.. sorry about that double post. Not sure how that happened.

A loose convertor perhaps. But I'd lean towards the driveshaft and u-joints first. Make sure you mark the shaft and it's oreintation to the differential. Make sure you get it put back in the same way. Scribe it with something or mark it with a can of spray paint. If you don't you get it back in the same way it will cause a vibration too.