Ok guys, Have a clunk from the rear end on selecting drive or reverse. Is it likely to be something simple like a worn diff mount bushing and would I see the diff move on a ramp?

Depends, I would be thinking more along the lines of a worn universal joint or possibly internal diff problem. If it was a diff mount bush you should have had some handling problems as well.

Has it come on all of a sudden or been getting worse over time?

63-82 Corvettes are notorious for wearing out the bushing that connects to front of the differential to the crossmember. If you never looked at it and the car has some mileage on it, good be that's what it is

Paul

LukesVette38901.7040740741

I'm wid Paul on dis one! The front diff mount is really bad about deteriorating, and causing clunks on gear engagements, and on take-offs. This would be the first thing to look at.

U-joints can also cause a noise, but normally, they make more of a "clink" noise than a "clunk".

If you put the car on a drive-on lift, you should be able to locate the source of the noise, no matter where it originates from.

Jotto, you beat me to it--I was getting ready to post a version of this
same question. My problem is half of yours--I get a significant "clunk"
when I shift into Drive, but not when I put it in Reverse.

The transmission has definitely been changed in this car; that much I
know for certain, since the housing itself is new. However, exactly who
did it, and what they did with/to it, I don't know.

So, Chips58, Paul and Joel...does that syndrome change your diagnosis at
all? Or am I probably experiencing the same front diff mount problem as
Jotto?

Same diagnosis here Diff bushing. $30 fix. Little more if you want to go with polyurethane. Although I tend to stay away from them for this bushing...

I say still, check the front diff mount bush first...

Thanks guys...will be on a lift by the weekend taking a look! Hope its the cheap  $30 fix rather than diff internals tho!!!!

I went with the poly bushing on mine and the clunk went away.

How difficult a fix is this? An 'up on the jackstands in the driveway' job,
with the usual hand tools, or something that would require a full lift/
specialized gear?