slow engine deceleration when shifting between gears. any way to stop this from happening?

There's a dash-pot type of affair that keeps the throttle plate from slamming shut. The slow closing throttle burns the fuel mixture cleaner. I've noticed it too with my manual tranny vehicles.

well if you rev it up in neutral it drops like it should but if its in gear it has that funny lag before it starts dropping. so i just wondered if i could change that somehow

their is a switch under dash that tells the pcm clutch angle ,connected to lever arm,remove and clean or replace,maby cure rev issue

Possible vac leak, tps or iac issue. Check your DTCs and report back.

all systems are good no codes or leaks. ive driven a few of these trucks and they all do the same thing so i think its supposed to do it but i dont want it to.

They are designed to do this as part of the emissions control system. It is, in my opinion, plain dangerous in certain circumstances. There is an aftermarket attachment that can be fitted that does away with the throttle lag. I makes a huge difference to throttle response both when you add and when you release. Any good performance shop should be able to steer you in the right direction. Failing that, check LMC.

do you know what the part is called? cant seem to find anything

I've driven a few of 5 speed chevy's and they all have done this. The 5 speed trans in these trucks isnt designed to be quick shifted, It normally holds rpm's up after clutch release for only 1 sec. It's normal operation and will only be a problem if you try to float gears or something. It's not a throttle lag as if still in gear it should still decelerate as soon as you let off the pedal. If it doesnt then i'd worry about it. Also when cold it will have a high idle and magnify the effects of this system.