The fore/aft movement should be handled by the rear control arms,have you checked the free play in your differential?
You sure all your u-joints are good?
I'm starting to suspect the diff.

 

I mentioned earlier in several threads/posts that I replaced the two-piece iron shaft with an single oem aluminum one off a 2000 SS. I also just dressed it up with two new spicers last week.

After reading tons of posts, I kept seeing several on re-positioning the rear ujoint 180 degrees for some vibration issues. I don't think it applies to my symptoms though but maybe?

 

I have a slight vibration in mine. it was worse but I have replaced everything. best progress was replacing the engine mounts. they was bad enough to allow the front of the engine to drop 1/2 to maybe even a inch. I assume this messed up the driveline pinion angle, check my sig for all the stuff I changed. stupid vibration is still there just barely. that clunk may be in the transmission. you auto or manual?

 

Its an auto.

 

The odd thing craby is that the same clunk has been there since I got it running again after a 13+ year hiatus. The old trans mount was a shredded mess. It's nice to have the one piece aluminum w/ fresh ujoints of course but, given the exact 45+ mph coast then light acceleration clunk, I could've saved a lot of time and $ to still be stuck at ground zero.

I've never dealt with a definitively bent or out of balance shaft. I always assumed that a defective shaft would manifest its symptoms in a more predictably and obvious way. To me, my scenario just seems so precise like it should a no brainer. It's maddening. And, like you, and SO many 4th genner's, 90% of stuff that ain't broke gets "fixed".

It seems like there are a fair amount of engineering flaws on 4th gens, some more than others, but I'm pretty sure all of them have been exposed by now, with very comprehensive documentation for solutions. 99.9% of the fbody problems are problems that don't require reinventing the wheel. And, the most common issues occur on millions of other cars as they also age. I understand the Venn nature of troubleshooting, it's truly my Achilles. I can replace and fix just about anything but it doesn't help that my adhd pushes my brain in too many simultaneously frustrating directions and muddy's up my troubleshooting assessment's even more.

Back in the day, I could stand under my car on the lift at my local garage while they were fixing something I didn't feel like doing at the time. I felt like a kid in a candy shop. I could grab and twist suspension and driveline parts front and back, spot leaks, etc. and my guy didn't care. It's amazing, the liberating perspective you have while standing under your vehicle versus lying on your back in a gravel driveway on a 5+ degree incline.

Today, I'm gonna poke hard around the rear end as the clunk seems to be more pronounced there. I'm also thinking of blocking up the whole vehicle again and taking a video of the drivetrain as it reaches the higher speeds. I know the dynamics are different without the normal weight load on the rear end but maybe something obvious will pop out.

I usually have the frame sitting on cinder blocks with a chain/come a long chained to front end so I can raise the rear end off the ground as needed-don't trust chocks. I usually keep a floor jack under the diff with a 2x4 so the axle isn't dangling. Should I experiment by changing the driveline angle by raising/lowering the rear end at different engine speeds? I know the road contour doesn't affect the clunk "trigger" but I thought maybe something else 'pops' out and helps point towards the problem.

 

Got a few videos that reveal some strange things.

 

once you get the rear diff up work one of the wheels back and forth and see what happens.

 

The rear wheels have no more allowance than the normal slight side gear axial movement.

I had my son take it up to 45+ and even underneath with my ear a foot away from the diff, it was hard to tell when the clunk set in because of the loud exhaust and revving engine. However, when I held my hand on the axle I could feel the clunk pretty well. The gear oil has never been changed so I needed to crack the cover off anyway and put some fresh in it. It was also the first time I got to see my new alum shaft spin at higher speeds. It looks well balanced. I also used an old strobe light I modded with an extra 10' of wire for convenience.

As I said, I took vids of everything I did. I also wanted the audio of the clunk sounds. One thing I caught on video was a couple of pinhole size breaches in the straight exhaust pipe that was spraying a tiny water/condensation stream like the pipe was full of too much water. I know condensation in the exhaust is normal byproduct of combustion and, luckily, it is perfectly clear a non-oily, with no odor at all. I'll be tearing into the diff today to hopefully finally find the clunk source. I hope it's not too bad.

 

45 is about the time my torque converter locks up. I wonder if it's at the other end of the driveline in the converter. a clunk in the driveline can travel all along it from engine to rear wheel. just a thought. maybe someone else has some insight as I am no help with transmissions.

 

Yeah, that's why I couldn't definitively say the vibrato clunk I was feeling was at the axle, even with my hand holding it. Even though the sound is pretty obviously coming from the rear while sitting in the vehicle, even with my son's ears, it's less obvious while lying underneath. I really hope it's not the tranny though. I don't know how clunk-action that loud from either part doesn't translate to crippling the car much faster.

Is it "torque shudder" that your talking about because maybe a tranny flush or even just fresh blood may help if a seal isn't completely shot. Both the transmission and the diff are filled with fluid that's 29 years old. Grant it a little less than half of that time it was dormant, but changing both can only help.

Should I use synthetic for both? The manual says that a GM additive for the diff is necessary but I assume with better additives and/or general fluid improvements since the 90's, a good synthetic will be good enough, no?