96 Honda Passport/Isuzu Rodeo, 4wd won't engage, only halfway engages?

The 4wd system is lever actuated,
not a push button system. When I try
to engage 4wd it feels like maybe it's
only half engaged or it's trying to,
there is a low rumble and vibrations
in the steering wheel, and floor of the
truck. I have no difficulty shifting the
lever to 4H, 4L or 2H it's nice and
smooth. I've checked the relay under
the hood labeled (AXEL) and upping
engagement it clicks, so I imagine it's
receiving power. So what I'm
wondering is this, does 4wd system
use vacuum to engage the front
axels, or am I mistaken? Also what
else would cause this issue, maybe
the snap ring on one of the hub
assembly's? I had the driver side CV
axel replaced about a year ago,
maybe the forgot/ misinstalled it?
Thank you.