Well let me some of yours first! I have a 2000 Suburban 1500 2wd with a 5.3 that is LOCKED UP. It has 150,000 miles and I assume they are very hard and not taken care of miles. I stole it from a crack head mom that needed bail money for $1000. It was running barely when I got it. It was running extremely rich and throwing codes like crazy.
Someone had tried to pull someone else that was stuck, by using a chain. As usual in these cases the chain broke on the stuck vehicle and slammed into the liftgate and back window. Nice chain impression on lift gate, broken window, and bent bumper. Remember now these are crackheads. I replaced the liftgate (same color) and window from local pull-a-part for $100
Interior is intact but nasty as hell. Going to pull seats and attempt to deep clean carpet.
I decided to drive it a couple of days before digging deep into the rich running condition. I am afraid that I made a major mistake in doing that. I stopped at the local parts store to pickup some replacements for all the burned out lamps and have them read all the codes for me. It was running rough like it had been everything including oil pressure and temps appeared good. Shut it off, go inside and get my lamps. counter guy reads the codes for me, bad O2 sensors. Go to leave and when I hit the starter engine started to spin then a pop and starter spun free. Couple more attempts to start and all I got was free spin on starter. Broke bendix. Did the parking lot starter swap. Yuck never again. Guess what new starter just grunts. Crawl back under loosen starter bolts, starter works. Tried turning motor by hand with no luck. I was ill, mad, and wet as it had started raining by this time. Towed it back home and parked it till I feel like being beat up by a Suburban.
My diagnosis is that the overly rich condition washed cylinders down and possibly contaminated the oil. I believe that when it popped when I tried to start it, it spun briefly backwards, breaking the bendix on the starter. Either it is a piston locked because of rings or one of the bearings had spun due to contaminated oil and showed no problems till it spun backwards and overlapped the other half of the bearing.
TL/DR: locked up due to broke ring or spun bearing.
Sometime this summer I will get out there with a long breaker bar, pull the plugs, and try to free up motor by turning it in either direction. If I find that it is terminal, I will be going to local pull-a-part for another 5.3 but will also pick up a 4L80 transmission to be rebuilt. Do a gasket and seals and knock sensors change on salvage motor. Replace motor and transmission with a small wiring change and reflash. Then I can enjoy it will I do the originally intended mods. It has factory tow package. I haven't checked all the build codes, so do not know how equipped underside is.