Transmission or transfer case

I've got a 01 chevy Silverado extended cab z71ive
had it for about 8 months as soon as I bought it
realized it burned way too much fuel took to shop
an was told knock sensor an o2 sensors need
replacement as well as previous owner bored out
both cats an put back on idk if that could add to
thelose of power an fuel incre uased use. But now
she's been parked one morning going to work I
started her up like I do every morning sat fir a
moment then put truck into reverse an nothing
then i went threw all gears an nothing happend. Sat
fir a second tried it again and when I put in revers
this time I mashed gas an it jerked me hard as crap
an same in drive maybe heard a lil humming noise
and vibration when I got on the road it seems to
have quit well I got off the interstate an I heard a
loud chain noise it sounded just like a big metal
chain dragging on the pavement an then she would
vary go at all when I mashed gas basically Fred
Flintstones it to parking spot an then she wouldn't
even lock into park or any gear for that matter any
more well I had it towed an when I went out to test
it again it was locking in yo all gears like normal an
seemed fine just for a second one the road then I
heard outta. No were that same chain noise an I
want to say it only d does it during shift changing
an if I raise rpms it will quit in out but soon as it
wants to shift or I'm starting to go from stop the
noise starts again I barely made it home if I drove
her about 32 miles an hour it didn't make any noise
till like I say it time for gear change