Here is what I Discover, (no pun intended) my rear wheel has a large crack right on the shoulder. I worry that it may burst and leave me stranded. Is this safe to drive or better to put the spare on?

 

Depends on how deep it is.

Stick a tooth pick ot something like that in it and see how deep it is.

 

The deepest point is a little over 1 cm

 

That is a tough call, under a cm and I would say just keep a eye on it.
But over and that is a close judgement call, how soon were you looking at getting new rubbers?

 

New rubbers sounds good but at the moment, Benjamins are kind of low. Maybe in the next couple months or before winter. But they are just very worn.

I will keep monitoring it to see if it starts to leak. I just hope it won't burst when I'm at highway speeds.

 

You can fill it with Super Glue, but keep a eye on it and run it on the back of the truck if it is on the front right now.
That way IF it blows not such a big deal.