Have a 4 WD question for our experts;thanks

[77 ChevySport]

Need help on the following for a general 4WD question:

I will be trading my 2003 4WD Trailblazer(50000 Miles) for a new Silverado.(Shortbed) Everything was going fine with the deal until they
took my T/B out for a test drive. They came back and told me my 4 WD does not work. No warning lights are lit up on the dash. They (Chevy Dealer) said they test to see if working on a non-slippery surface (Summertime) by making a "Crazy Eight" with the 4 WD turned on. If it grabs it
is working if not >not working. The last snow (not long ago) it worked fine for me. Of course this will effect the deal we make. They think the
clutches are not grabbing now and a simple flush for new gear lube may work. If not they said it could need new clutches and would cost
about $1000 for the work. Is this true what they are saying? All comments are welcomed. Larry

P.S. I have never changed the gear lube in the unit; as the truck has a little over 50000 mile on it. Should the lube should have been?