Horrible Clunk - Troubleshooting help needed!

[BlueMonster]

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.

[maxwedge]

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.

[BlueMonster]

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?

[WEF]

Grease the driveshaft yoke going into the transmission.

[BlueMonster]

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.

[BlueMonster]

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!!

[BlueMonster]

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!