Tranny Help!!

[batwing]

Looking for some advice or insight. I have a 2004 Duramax with 101K. This past week the tranny appears to be slipping. If I take off from either a stop or a rolled stop, it slips and feels as though I'm in neutral and the truck just revs up. I will then put in neutral and then to drive where I will slowly accelerate, however it will not shift into next gear until I come to a stop, shut motor off, and restart. I took to tranny shop and they said that it's a second gear slip according to diagnostic tool and that it's not a sensor and that I need to replace tranny. They suggested a used tranny with 54K and would warranty for 1 year (parts and labor) for $1800. I thought these things were bomb proof. I've never towed anything and drive conservatively. Does this sound right to anyone?

Thanks!!