I just recently had my transfer case repaired, not 2 days later the transmission went out, is this a coincidence or a bad repair?

I have a 2001 Chevy Tahoe that just had the Transfer case repaired.  The day I brought
it home I was getting on the freeway, slowly due to traffic and heard not one, but two
"hard clunks."  I had only driven it maybe 10 miles since the repair.  It drove, I called
my mechanic and brought it to him the next morning and he said it looked fine.  Then I
drove it another 40 miles, when it clunked again and heard a loud whine.  It was tow
back to the mechanic where he said that the transfer case was fine, but now it was the
transmission.  They are saying that it is pure coincidence that that happened,
unfortunately.  My Question:  is that the case or was could something have been done
wrong when the transfer case was replaced.   second/third questions:  what is a
reasonable price to rebuild the transmission and approximately how long should it
take?  Thank you.