SierraDually

There's a kit you can buy to convert to 4l80e from 4l60e. Comes with stuff like a transfer case adapter and hardware,new filler tube, shifter cable bracket, wiring harness adaptor and A couple other things. Have to re-flash the ecm too I think. First gear feels a lot different. There's quite a difference in torque rating. And the 80 is a little heavier. It's worth it if you're planning to keep it and do some towing. For general street use as a daily driver to work or whatever, the 4l60 is adequate. i can go to the junkyard tomorrow and pull a tranny then list it on kijiji the next day. you get what u pay for. I would not put a $350 tranny in my truck. Around here you can get a decent 4l60e rebuild with warranty for around $750-$850. Give them your old trans and you will save a little. It matters what year it's from. There are valve body issues in some. Google " p1870" I think it is. That's one of the notorious problems. You want to get one that already has those sort of issues taken care of. Any decent rebuilder will be able to tell you everything you need to know about the one you buy. Good luck

On that motor the fuel pressure regulator is under the plastic intake manifold plenum. If it fails and leaks it can leak into mostly one cylinder and cause what you describe. It won't occur at driving speeds. Just at idle. Not saying that's definitely your problem. But can easily be ruled out with a 5 min fuel pressure test which I would do before throwing parts at it. In my experience, spark related misfires usually throw a code, but not always.

If I were you, and I'm obviously not, I would upgrade to 4l80e. It's a noticeable difference. But depends on what condition the motor and rest of truck are in. Probably cheaper to rebuild the existing. Just a thought...the warranty may not be transferable but if that tranny was rebuilt by a decent shop and has warranty remaining, maybe your family member that you got it from would take it in for you under his warranty. I had a similar experience but it wasn't a family member and it also wasn't a coincidence that he was selling the truck and the trans suddenly started acting up. A lot of guys will throw a bottle of Lucas transmission fix in there and get another 1000km from the tranny which is enough to sell it to some poor unsuspecting buyer. Then it burns up completely a week later going down the road. I'm not saying that's what has happend here. If you got it from a family member they probably wouldn't do that to you. Are you getting any codes regarding the poor trans performance or slipping?

What year is your truck? Are you getting a flashing check engine light during misfire or any codes stored?