Need Help with Moving T-Case Shifter

[MJM]

Hello, All
My 1980 FJ40 non split case T-Case shifter has a mind of its own it drifts back while driving and you know what that means, grinding. Anyone know of a simple adjustment so that it will stay put in 2H and not move. Thanks in advance.
NESTA

 

[cruzer]

bungee cord

 

[tobydog]

Vibration ?

If not maybe a stuck detent or spring.

 

[Pin_Head]

1) Missing detent ball or spring.
2) seriously worn shift fork.
3) misadjusted linkage rod.

 

[MJM]

I will take a look under there and see Just wanted to know what to look for.
Thanks