Horrible Clunk - Troubleshooting help needed!

Hi everyone. I have a 1990 GMC Sierra 1500. It's rear wheel drive and for the past month or so, I've noticed that I'll be sitting at a light and all of a sudden I'll hear this loud clunk and it feels like if the truck got hit from behind. I thought it might have been the u-joints, so I replaced them both, but the problem still occurs. Now it's gotten worse. Even when I am driving at highway speeds, at random I will feel this hard clunk. It almost feels like something is binding up and lets go right away.

I am not sure what it could be. The transmission fluild is topped up, and like I said the u-joints are new. The drive shaft actually is only 4 years old as well, as are the gears in the rear end.

Can somebody please shed some light on what's going on here? I am not sure what to check on this one in order to figure it out. I am starting to worry that I'll be picking up pieces of my truck on the highway really soon. :uhoh:

Do you have the Eaton locking diff, G80 code on the tag in the glove box. Lots of things can do this but this a common area for your symptoms, if you have the G80 diff.

Hi maxwedge. Thanks for taking the time to reply. I checked on the Eaton diff and that's not it. My truck does not feature the G80 code.
Have you any more ideas?

Grease the driveshaft yoke going into the transmission.

Hi WEF! Thanks for the response! I will definitely give that a shot.

Just for my own understanding, how does the lack of grease on the yolk affect that? I didn't realize there was anything for it to rub on other than the rear transmission seal.

I'll let you know how it goes.

Actually, I sat and thought about it for a moment and I can see how the lack of grease there would be a problem. Hopefully I will get a chance to do it tomorrow after work.

Thanks again!!

Thanks so much for the help. I greased the yolk and while I was under there I noticed that a hanger bracket for the exhaust was broken causing the muffler to knock against the passenger side shock. I fixed that as well and the problem seems to be gone. Hopefully it stays that way!