2002 4x4 Tahoe

My 2002 chevy Tahoe ext was having trouble
starting and i would have to jump it off sometimes.
I replaced the battery on 7/11/18. Was fine for a
while. About 4 months later it started doing the
same thing. I took it to auto zone and had them
test the battery and alternator.  They said it was
fine, but replaced the battery since it was still
under warranty. Now 2 months later it won't crank
at all even if we try to jump it off. The lights and all
interior works, but won't even turn over. I bought a
new starter last night. Haven't got it put in yet ro
see if that works. If it doesn't, what else could it
be?