Transfer case problems

My chevy silverado 2001 1500hd has a
problem I cant figure out. I replaced alot of  
sensors and lines and even a new
transmission plus cv axles new cluster
feed and it still is running bad. First of all
my transfercase engages in 4 wheel drive
but it pops and grinds really bad and I can
only use it for a short time before a light
comes on amd says service 4 wheel drive
I checked fuses replaced the cv axles put
a rebuilt transmission in it. Also When I
am in park or neatrul my speedometer
goes to 20-45 miles and hour but idles
normal. I replaced the cluster thinking that
was the problem its not that and a couple
sensors. When in drive it starts jerking the
speedometer at about 17-20 miles and
hour unless I put it tow/haul then it doesnt
jerk. Someone please help I am definitely
out of options right now I mean I cant
afford new parts or anything like that right
now but whatever will help maybe get a
list going on what I should purchase when
I have the chance