I have a 1984 Camaro with 305 LG4 a 700R4 tranny. The car needs a tuneup and it idles at 850K (I know thats high). I know that I need a new shift cable bracket as mine is bent and the cable wiggles a lot. I have an issue with it also slipping out of park into reverse because of this.

However my issue is that when I do put it in drive or reverse from park it makes a loud and noticeable clunk when it goes into gear.
1) Could this be just the shift cable
2) Is it a tranny issue
3) Is it a driveshaft issue
4) Idle issue

When driving the car shifts perfectly, only at idle does it clunk.

 

...Or axle issue, or the trans mount, or a torque arm mounting. Or something else. You're going to have to get under thare and take a look to be certain.

 

it might be the u joint...
i had that problem around 280k miles