2003 Silverado 2500hd extended cab short bed. 240k miles. One day I pulled out and it shifted from first to second then slammed into first and was stuck in first for the rest of the drive. Hooked the tech2 up and got codes ; p0776 and p1621 and service brake system came on. So I figured the trans was on the way out so I bought a used built trans and before I put the built trans in I put all new speed sensors including the transfer case, I put new NSBU, went through the entire trans harness, even taking the loom and tape off. Checked every pin and every connection including tcm and ecm. Truck is still having the same issue. I’ve had the harness on and off a hundred times hoping I missed something. But it does it intermittently. But when it does shift up through all five gears it is harsh and downshifts aren't pleasing either. I have reset all taps and have tried fast learn. Nothing works or helps . Fluid level is perfect and new along with both filters . I also replaced internal harness. And also replaced tcm. I’m at complete loss , hopefully someone has the cure. Mike L hopefully you can help
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