It looks like this may be a fairly common issue with this transfer case but not in the way you might think. The price they quoted is a bit high it seems around 600 is about what most Ford dealers charge for this job. However there are reported problems with corrosion in the actuator pin that they actually moves the shifting fork. This pin goes from the Vacuum motor to the shifting fork inside the transfer case. Check out this thread from a another F150 Forum I hope this helps.
Incidentally I would suggest a subscription to Alldatadiy ( http://www.alldatadiy.com/index.html
) it is sort of an online vehical specific shop manual. I have long had a subscription covering my 1995 F150. Your "manual" has all of the service bulletins parts break down, wiring diagrams etc. One of the things that is has is a listing of job difficulty and standard time for a given repair. This is what some shops use to figure a price especially if they are estimating what a job will cost. Knowing what the standard labor time is supposed to be can give you an idea if the shop is padding their labor costs or not. This is money well spent.
I looked up this job for my 1995 F150 and the skill level is B and the repair time is listed at about 1 hour. This is for the removal and replacement of the shift actuator. The shifting fork will require the removal of the transfer case. The $900 dollar price quoted by the shop you mention is about what some places are selling refurbished whole transfer cases for.