98 JIMMY 4.3 V6 125000 MILES

GMC C2500

My jimmy has started revving up rmps and not shifting until 30mph(1st-2nd). After that, it is fine. It will go through the rest of the gears fine and from 60mph down to 45mph just like normal. But always from a stop it will rev up and I'll let off the gas and give it some more and it will shift to the next gear. It will shift through the gears just fine with no hard/jerking problems past that. The check eng. Light is off. I have replaced the cat in the past few months and have had no problems with that. I also had just taken a 300mile trip the day before this problem started. I don't know if this a tranny or fuel problem. Any help?
June 18, 2007.

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