I have a 1986 2+2 5 speed but when I put the car in reverse it clunks really bad to the point that it actually comes out of gear and into neutral. Does anyone know if this is a transmission problem or rear differential problem. Anything that I should try. Car was sitting for 10 yrs, but I know I had this problem before I garaged it. Thanks in advance.

Check the trans fluid, if it were in the diff it would happen in all gears. The fact that the car sat for so long I'd suggest changing the fluid in both as cheap insurance. If the fluid doesn't help, you might want to check the clevis on your new master is adjusted properly, should be just a hair of slack before you start pushing fluid. Could also be a bit of air int he hydraulics you didn't get bled out, these can be a pita to get right sometimes.

Ok. Thank you. It was like this before I garaged it so hopefully this will do the trick or I will need to take it to a shop.

I have the sane problem, discovered sub frame mounting bolts rotted right through the floor on both sides. And that's from sitting for 10-15 years. Remove the carpet and take a look behind the driver and passenger seats and good luck.