I have been given 2 trucks do to a relative's death.  One is a 2001 Chevy Z-71 and the other is a 1992 Chevy S-10.  The Z-71 has a 150,000 miles on it and the S-10 has 170,000 miles on it.  Neither one has ever had a transmission service.  I was going to service them but have heard that after going so long without servicing the transmission I should not.  I have heard that it seems to burn up or tear up the transmission after doing so.  Please give me some advice.  

I would do the service.  The members of this forum tend to specialize in hot rods, though.  You might want to seek out a forum for a 4-wheeler magazine or something similar.  

Do a fluid and filter you will only drain out half the fuid anyway, check the pan for eccessive clutch material or filings aluminum or steel and DON'T use seal conditioner it turns the seals to mush. Use Mercon Dextron III fluid and a cork gasket if posible.  

The trans in my company truck (92 Chevy 2500, 700R4)went 240,000 miles without a service. Ever. It started slipping one day, so I told the boss about an old trick used car dealers used to do back in the day on Gm autos that were starting to fail. Put type F in them. Honest to God, I used my vacumn can to pull out several quarts through the dipstick tube, replaced with type F, and that thing is still going, now with almost 300,000 on it. Type F is thicker than any dexron, so it boosts line pressure, which can be helpful in a worn transmission. Just like a tired engine, that can't maintain decent oil pressure anymore, just use a thicker lube!