Yeah don't fall victim to a parts tosser. Now it's on them to fix it properly, but how is a lack of vacuum causing this noise? If it is the 4wd system partially engaging to make this noise that means it's naturally engaged, which makes no sense.
When climbing a grade engine vacuum goes to near nothing, in some vehicles (including Fords) there's an auxiliary vacuum pump that will engage to keep the vehicle supplied with vacuum.
What are the odds you have that pump and it's noisy?
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10