wow. this one is a mess.
so so many things this can be, and it all doesn't really sound related to me.
you really need a tech 2 for these trucks. the big thing is the freeze frame data. when they set a code, they capture a snap shot of everything happening at that time. if you have a scanner that can pull up that data, that will got a long way. most high end scanners can do it. the clone tech 2 being the cheapest at about 300$. and then a monthly subscription to repair software, like alldata or prodamand will help digging thru how all the systems work together and tests to run for each code.
sadly you're going to become a troubleshooting mechanic to keep it. shops are all terrible with these trucks, well any real electrical troubleshooting at all, they mostly just empty your wallet and leave you with a still broken truck.
just off the top of my head, my guess is the stalling 2 years ago was probably a hybrid cell voltage over volt. a freeze frame would have shown forsure, but as the hybrid battery gets weak, the cell voltage is less stable and swings higher and lower. if during regen any cells go above 20.8v I believe it is, it opens the main contactor in the hybrid battery. this basically kills the entire truck. except some 12v accessories. that's my guess. hybrid battery really only last 100k or so in these. by 130k they are acting funky. bucking jerking and all kinds of stuff. the number of guys that got sold a new tranny when it was a weak hybrid battery is just depressing.
the new stuff, few different things come to mind.. the can-bus crashing from that rear system module getting corroded out is common in all up north trucks, ours just rely on the network more than normal trucks. youtube channel "south main auto" has a good video on trouble shooting and bypassing that module if you can find it.
next is wiring harnesses rubbing on things, there's a few different gm technical service bulletins about different spots to check. but down by the tranny there's a big plug going into the tranny that can rub on a heat shield next to it and short out wires. the other, and I think it was fixed by your year truck. but easy to check. there's a thick harness that is attached to a bracket about where the normal truck's alternator would be. the bracket is metal and sometimes the harness can wear on that bracket and short out stuff.
watch out for these trucks weird cranking. they don't have a starter. they use the tranny front motor to spin the engine. they will spin it over so fast a lot of people think the engine is started and then stalls, but it's never started. Just being spin over at like 1000rpm for a while till it times out.
limp home mode you saw is a weird one, I've never seen anyone post about that or experience that. my guess is that was just the normal v8 engine limp mode, usually something like a throttle body or throttle pedal not agreeing with each other will throw gm drive by wire vehicles into limp mode. so that again sounds like harness issues, not really hybrid specific issue.
for now if it's running and driving, you pretty much need a good high level scanner to work with thru issues. if it's not running that south main auto youtube has some great videos about checking the can-bus network. you kinda need that up and running so your scanner can talk to everything or you're just blind guessing.
oh the tapping under the truck could be lots from things. even a torn motor mount. all these Gen trucks tear motor mounts all the time. there's a huge sticky thread about it. mine had a weird sound at idle in neutral that went away in drive or under load. kinda sounded like timing chain slap. turned out it was just a bad motor mount setting metal to metal.
good luck with it. it's a bit of a deep rabbit hole. one guy on another board had just remote started his truck to warm up after a ski trip. ran for a bit and then shut off. took him 6 months, tons of ideas from everyone. he even ended up buying a 2nd truck he found cheap that needed a hybrid battery and rebuilt that himself and then bam, he finally found it. Just before he bought it, a reman engine was put in it over the afm lifters failing. well turns out the shop didn't know you can't use standard pag ac oil in a electric compressor ac. pag gets conductive over time and the hybrid module saw voltage where it shouldn't have and opened the main contactors for safely.
I only tell you that cause that's the kinda persistence you need sometimes. no shop mechanic is going to find that for you. that's an extreme case, and a eazy fix once found. flush the ac system and refill. but yeah, these have a lot of systems and safetys all running thru the standard horrible gm wiring. battery cables tend to rot from the inside out and catch everyone out.
