need help got a shudder

[wintimidator]

A week ago my truck started shuddering at part throttle above 30mph and it evens out above 50mph. When I get on it half throttle or more it goes away, when I coas it goes away. Truck has 69K miles, hadn't changed the diff fluid having owned it since 8/2011 so I pulled the cover off and cleaned it the gears look new not a scratch on them. Refilled it and drove it, same problem. The U joints have no play, and there is a rhymic squeak coming from back there at 4mph or less so I think something is rubbing while spinning. Anyone have this problem before or ideas what it could be?
Thanks

 

[92GMC]

even though the ujoints are tight I would say that's the squeak. try greasing them.

 

[JonZ71]

Sounds more like a trans issue

 

[wintimidator]

I really hope it isn't the trans because I can't be immobilized for more than 8 hours (when I'm sleeping). Is it likely to be bad after 69K miles though?
The squeak sort of went away, it's intermittent now