I'll second what Kenny said. Myself, i wouldn't call it "rare". But it depends on how you look at it... Its also true that 91 was the last year of the "square" body style, and was the first year of the 4L80E trans which they used at least through like 2009 in express vans at least. Dont quote me on that last part, but its probably roughly accurate.
It is a VERY nice truck! And there is no harm in asking what you are! But i think realistically, you will be lucky to get over $7,000 for it. Good luck! Its a very nice truck!