I guess I should expand on why I think its a bad u-joint or possibly the tranny mount.....
When your foot is on the gas, you have "load" going through the drivetrain (which includes the transmission, driveshaft and rear end). When you let off the gas and/or brake the "load" disappears and the drivetrain is basically free wheeling/spinning. Your driveline angle also changes in this condition. When you get back on the gas, the driveline angle changes again, load is transmitted through the drivetrain and a bad u-joint will make a clunking noise during this transition.
A bad/cracked transmission mount acts much like what I described above in that it will move up/down with the "load" being applied and taken away.
Look for signs of grease leaking from around the u-joints caps. Check to make sure there is no play in the u-joints. In other words, grab the driveshaft near the u-joints (both front and then rear) and make sure it does not move up/down or side/side. If that checks out, I would remove the driveshaft from the rear end pinion yoke and make sure the rear u-joint is not binding and moves freely but smoothly back and forth. You can lift up and down on the rear of the driveshaft to check the front u-joints, this way, you don't have to remove it from the transmission tail housing and risk dumping fluid all over the ground. If you decide to remove the whole driveshaft, I found that a 20oz plastic bottle of soda (empty of course, your choice of flavor
) makes a nice tail housing plug. Simply cut the bottle in half, stuff a few rags into the bottom half, slide a hose clamp over it and then slide the bottle over the tail housing and tighten up the clamp to hold it in place. This will catch the fluid. If you prefer, skip the rags in the bottle part and that will let you know how much fluid you actually lost during this and you will then know how much to replace once you have finished.
As for the mount, that should be obvious....make sure it is not cracked and dry-rotted. If it looks good, try putting a floor jack under the tail housing and GENTLY lift with the jack. The mount should not have any up/down play in it at all.
I'm not always right, but this is where I would start looking.....