Few Quick Questions (4wd)

Ok, the last few months the 4wd on my 97 rodeo will engage fine, but when it comes time to put it back in 2wd, nothing. I found that if I reverse for about 50 feet, it cuts off.
I was wondering if their might be a sensor or switch i can clean to try to make this more responsive?

And while I'm here, my parking brake has lost alot of it's tension, how do I adjust this?

Thanks