Idle might be too high. U-joint might be toasted. Could be a simple leak from shift seals. Get under car to see if you can gauge what area the leak originated from.
sounds like your clutch for the reverse is going and one of the seals are bad. good idea to do is change the fluid in the trans. see what color the fluid is in it, also smell it see if it smells burnt. Also check to see if there is any small fragments of metal. there is some adatives you can also put in it to make the rubber swell and stop it. it's also good to change your fluid too depending how long it's been
Fluid leak is from tranny.
Clunk is from clearences in the rear end...
..jack it up, put it in neutral and roll the driveshaft back and forth.
Imagine it now with the engine (instead of your hand)turning the driveshaft.
Easiest way to minumize clunk....take foot off brake before shifting into gear.