This is a fun thread.
1. have compression check a.) as is(DRY) b.) with oil in cylinder "WET"
a.) will tell you the current condition of engine with rings as a question.
b.)will seat the rings with oil and raise compression readings. if you have a bad injector it will most likely wash the cylinders down lowering compression.
while compression test is done the glow plugs are out. install all 8 new($160) and anti sieze the threaded portion. (save on labor)
2. GM has always been a 60,000 mile magic mile mark! the diesel is Japanize though that is typically a 100,000 mile magic mile mark!
60,000 change idler and tensioner pullys($50 idler $68 tensioner and pully),check alternator($180-200 new), change serpentine belt($45-70), water pump change(Hitachi new $200),change thermostats($40 each),check all hoses and coolant pipes replace as needed, flush and fill. reservoir cap($15) 15 psi cap.
3.) road test for a while after clearing all codes you will most likely be fine. if it is not then its head gasket time. *unless compression check reveals head gasket right off the bat.
4.)head gaskets: remove both heads send to machine shop pressure check and plane(call shop for price), intake gaskets, and fuel injectors($1600 new $1600 core charge)
* do not just slide head gaskets in.
If you can do your own work. checking and replacing what is broken is great.
if you pay labor than having the parts on hand saves labor if you are at an hourly shop. flat rate shops may charge for each replacement item(costly)
just my experience with many GM trucks(gas and diesel) on the hook of the tow truck. and most are towing something when this stuff craps out.
BTW injector pump is about $842
Magic mile mark is when things break. not all the time but most of the time.
keep old stuff belts pullys etc. helps in the event of a break down and parts are not available.