Physical clunk from somewhere on driveline

[baloneyhomie]

Hi all, as of recent I've been experiencing a strange clunk with my car. This only happens going highway speeds, and most noticeably around 65mph. What happens is that if I'm coasting down the road and press the gas, I can feel a fairly strong clunk as if something is physically moving. Once the clunk happens the car accelerates normally, but it definitely feels like a delay in power delivery. This only seems to happen in overdrive. If I hit the gas hard enough to force it to drop down to second (of which I rarely do - I usually just turn off O/D to kick down to third) it doesn't clunk.
I have yet to check the driveshaft U-joints, but I do plan on doing so when I get around to doing the transmission fluid soon (of which is bright red but smells a bit burnt - it was last changed a bit over 20k miles from the PO but that was back in 2007) and differential oil. The car is currently almost stock, the only thing that would affect driving would be the Marty tune I have installed, although I can't recall if it did the clunking prior. I've tested the engine mounts too and they appear to be in fairly good shape given its age and low miles (43k). The transmission is not J-Modded (yet)

A buddy of mine has a 2002 Grand Marquis LSE and he seems to experience the same issue as I do - perhaps it is related to the age of our vehicles since they're within a year of age. Fingers crossed it isn't related to the transmission.

If I have time I may try and get a video for y'all to better describe my issue.

Thanks.

[offroadkarter]

Most likely, you're feeling gear backlash in the rear differential.

[blazen71]

I'm wondering if the clunk is present in overdrive because the torque converter is probably in full lock up?

[Mr. Man]

Possibly a bad u-joint. Also check for a loose or broken suspension part.

[Comin' in Hot]

Rear upper control arm bushing??

[MyBlackBeasts]

Rear upper control arm bushing??

That's what I'd look at closely.

[justbob]

Rear upper control arm bushing??

I’ve annihilated 2 complete sets and never heard any clunking. Just a seriously unstable dog walk feel..


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[baloneyhomie]

Possibly a bad u-joint. Also check for a loose or broken suspension part.

I wiggled on the driveshaft when I had the car on ramps; it's solid. Didn't wiggle at all.

I've gotten the parts I need to do the tranny fluid and rear diff fluid in so I'll be doing those this weekend. I can inspect the rear end and see how everything looks.

[daniel.drives]

I think that's a feature of the 4R70/75, it's happens on both my 08 vic and less so on my 03 marauder. Same exact conditions you describe, cruising at 65ish and you lightly get back on throttle