Re: 3/4 ton blazer/
Look at 2 things: if it still has the badges on the side, they will say 2500 if it is 3/4 ton... and look at the number of lug nuts, if it is 5 or 6, it is a 1/2 ton, if it has 8 then it is a 3/4 ton.
These are not rare at all.
the best bet is what tobiahr said, check the VIN to know for sure.
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91 Chevy Suburban Tonawanda 454 R2500 - SOLD!!! 
04 Dodge Durango 5.7L Hemi 2WD, 24K miles (as of July 2011)
In the past 30 years about 90% of Fords are still on the road, the other 10% made it home.
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