Vibration at rear end. U-joints?

[MatBirch]

My 97 2wd has an on again/off again vibration while accelerating from low speeds. I can settle it with a simple throttle lift. It also has the telltale clunk on take-off. Maybe 2k miles ago, I had a mechanic check it and he said the u joint was fine, and didn't know why it clunked.
Should I just change it ? Truck has 180k on it, with no knowledge of if it may have ever been serviced.
Thanks for any insight.

 

[SenorNoob]

It's probably due at this point. But I like to know what a problem is before changing parts. Get the rear up on jack stands and rotate things by hand to find it.

 

[Uncle Gump]

Be sure to check the rear trans mount.

Bad u joints tend to be bad all the time. Not come and go...