2001 chevy silverado the sevice light is blinking one side 4 ciliders only seems to be working?

well the problem is they had 4 or more radio packages in the car new.
so is not ONE ANSWER EVER
the sevice manual covers all this in sound/audio/entertainment chapters.
what is a 2500hd
silVerado 2500 HD.
Entertainment Systems
3 BASE AND 3 UPLEVEL VERSIONS THERE 6 TOTAL OR MORE.

BINGO ON BASE. 1 OF 6 +

https://imgur.com/a/ctEJUb1

Sounds like your brake light switch is bad. It is located on the brake pedal arm.
Amazon com ACDelco D891A GM Original Equipment Brake Light Switch...

you should have 4 freeze plugs on the block 2 on each side

Chevrolet / GMC / GEO Transmission Look Up Table

www.autorepairmanuals.biz/page/372807

Blazer, 1993-, 4L60-E. Blazer, 1994 - Up, 6.5L V8 Turbo Diesel, 4L80-E. BLAZER/TRAIL BLAZER/JIMMY, 1995-2003, 4 SP RWD/4X4, L4 2.2L L6 4.2L V6 4.3L ...

Transmission Applications - GM Automatic Transmissions

www.gmautomatictransmissions.com/transmission_applications.php

Avalanche, 2001-06, V8 5.3L 5.7L 6.0L, 4 Spd RWD/4X4, 4L60E. Avalanche, 2004-06... Camaro, 1994-05, V6 3.4L 3.8L, 4 Spd RWD, 4L60E. Camaro .... Monte Carlo, 1997-05, V6 3.8L (W/S.C.), 4 Spd FWD, 4T65E. Monte Carlo ... Silverado, Sierra,1500-3500, 1995-06, V8 4.8L 5.0L 5.3L, 4 Spd RWD/4X4, 4L60E. Silverado ...

4L60E Transmission Rebuilt ' eBay

Hello, my name is Ben. Your starter is down low on the passenger side of the truck, at the right rear of the engine. If you get under the truck on the passenger side and look up, you will see it.

IF YOU SEE THE WORDS "THEFT" AND A RED LIGHT BLINKS THEN YES IT WILL REQUIRE A CODE ONLY IF YOU TRY TO PUT IT INTO ANOTHER CAR. IF YOU JUST UNPLUG IT, IT WILL STILL WORK WHEN RECONNECTED.

Sounds like the chain in the transfer case has broke or the linkage is out of adjustment

its in the engine compartment in the fuse box on the drivers side next to battery

If this is the kind with 4 holes in i, then you need to get the removal tools or use 4 nails or long screwdrivers all at same time and pull out the stereo.
If you do not have the holes, then you may have the kind that requires tools to slide in sides between stereo and bracket to loosen the hold. A butterknife sometimes works for this..

did you turn key to on position and depress throttle 3 times