305 engine? Should be 302 unless its modified in some way I'm not aware of.
The 87 Bco has a shift on the fly t-case. As long as all your wheels are going at the same speed you can engage 4x4 high at any time.
- Lock the Hubs
- Switch to 4 high.
- enjoy
In the winter time I leave my hubs locked and drive around in 2wd HI till I hit something that need more driving force.
For 4x4 Low you must
- come to a complete stop,
- shift to neutral,
- Move the t-case to the 4x4Low position.
- Then you can place the transmission back into gear.
To take it out of 4L
- Come to a complete stop
- Shift to natural
- Move the t-case selector to 4hi or 2wd
- shift the transmission back into gear
Simple once you know what the steps are.
You noted that yours grinds when your trying to shift, I would inspect the shift linkage to make sure it is all in the proper position.