Gear shift problem

1985 K-5 6.2 Diesel - Gear shift lever on steering column is moving about freely. Won't lock into any gear. Engines starts and runs but when trying to switch to and gear (Reverse, Drive, 2nd, 1st) just moves up and down freely. Any suggestions where to begin?

Will it lock in park when you pull back on gear shift?
Adjust and Check all of the likage for being lose.

If no help pull steering wheel and check the locking plate and dog notch on shift tube.