I got 03 avalanche that's been swapped over to 6 spd manual and 8 lug 2500 rear end, after getting up to bout 70 mph on interstate felt like jumped outta gear w some noise, I'm now stuck n 4 low, an

So you have a transfer case problem?

Yeah it would appear so, I just bought it and knew it was custom just don’t know the extend, pulled the fuse , got to disengage, same problem everyone else has , something electronic, every time I turn the key on I get something diff on 4wd selector switch, I don’t know if had the right transfer case, clutch or flywheel? There is def some slack n the driveshaft or u joints, but some noise, let out on clutch sound like brakes scrubbing and same as moving and coming to stop but it’s right there in the floor by shifter, advice? Also posi rear end

So you bought a kludge made of mystery parts and it does not work correctly. I suggest finding a good 4x4 shop and have them take a look at it and identify all the parts for starters. Maybe they can figure out what was done wrong.