4wheel drive?

I have a 1988 4runner, it has a 22re 4cylinder and automatic tranny. Everytime i go 4x4ing the 4wheel drive linkage come out. Right where it comes down from the shifter it keep popping out the pivot. Any ideas on this or how to stop it?

I'm not familiar w/the auto t-case, but something in the linkage may be worn (bushing, etc), or the mount for the t-case to the cross member may be shot allowing the t-case/tranny to flop around, causing the linkage to move.