We've been overlanding in stock GM vehicles for many years. The GMT400, GMT800 and GMT900 series trucks and full size SUV's easily handle mild off-roading and eat up highway miles in comfort. As BritKLR mentioned, "Parts and service have always been reasonable and easy to work on." I'll add that we've never had any issues with the GM IFS 4wd system, on trucks with minimal modifications to the suspension.
As for the comments re: limited aftermarket support. Not true. You can get suspension lifts, aftermarket bumpers, roof baskets, etc. For interior storage and organization look at Fleet and LEO upfitters.
The Suburban/Yukon XL has been the best platform for our family of 5 +retriever, for daily use and week long unsupported expeditions. The Burb gets used daily and weekends & holidays as our expedition vehicle into Anza Borrego, Big Bear, Johnson Valley, Death Valley, Colorado, Utah, etc. The last pic is a weekend day trip to Anza Borrego, Dos Cabezas Railway station ruins. We also backcountry camped all over Death Valley. Our favorite spot is eye of the needle in echo canyon.
1985, 1990, 1994, 1997 Chevy/GMC Suburban 1500 4x4's: These were the trucks that my parents owned and I used to haul friends to the lake and ski trips to Colorado. The only mod these trucks received was 31" Goodyear Wrangler tires. These trucks definitely set my bias for GM trucks as excellent platforms for family "expedition" use. The '90 suburban was capable of clearing CO state highways of hood tall snow drifts with ease, no plow.
I have owned a Ford Bronco II and Toyota 4Runner. Both had major mechanical issues that standout because they required a tow. Ford used CV joints on the rear drive shaft. The rear CV boots wore through every 10k for the 60k miles I owned it, due to the severe angle of the stock driveline. The 4Runner was a 1990 V6 and blew head gaskets constantly... still got $10k for it in '96 with 110K on the odo. Yota's do hold their resale value even with known engine issues.
My personal vehicles:
2000 GMC Yukon XL 1500 4x4 XLT: Expedition ready off the lot. The only modification was leveling it with a torsion bar crank to fit Michelin LTX 285/75-16 tires... on the ugliest chrome typhoon wheels I've ever seen(PO contribution). It handled the terrain (rocky hill climbs, sandy desert washes, rough, rocky mining roads and blasting down forest service trails) we subjected it to well. Sold with over 250k miles and no serious mechanical issues. I did have to replace the trans at 218k. My fault, hit a wet spot on slick pavement accelerated for "fun" G80 engaged at the same time the trans 1-2 up-shift, broke the low/rev sprag when the rear tire hit dry pavement... still drove it home and to the dealer on Monday. Other than the trans I replaced the rear "auto-ride" air shock suspension system twice.
2011 Chevy Suburban 1500 Z71 Factory G80 w/4lo (not all GMT900 series burbs have 4lo): No lift... yet. 265/70R18 (32.5x10.5) Michelin Defender LTX M/S Tires. I've also built a 2 drawer system for the rear cargo area. Works great for expedition camping. No reliability issues. Has a few of the usual GMT900 platform issues; Cracked dash and uses oil excessively IMO. GM says it's normal for the LS to use 2 quarts in 4K miles with the AFM system. We completed the White Rim Trail in this truck. What I found interesting about my experience on the white rim is that the jeep(Barlow Jeep Tours) out there were running 1 ton drivelines with 37-40" tires making them much wider than the burb. I had to side hill the burb several times on hard scrabble hill(steep rocky switch backs and shelf road) so they could pass on the outside edge.
1997 C1500 ECSB 2wd, 277k miles: I've owned this truck for 21yrs. Used it on the family farm, lots of long distance travel, towing, camping, etc. I still use it to haul our dual sports to OHV parks and trail heads. I used it last weekend to scout about 20 miles of forest roads that are severely washed out and rutted. It does have a 2" level lift, 31" tires and Auburn limited slip rear. Those mods help a lot off-road. I've taken it through some very rough gas line roads in the Ord mountain area. 4wd side by sides were getting stuck on a steep rock section and the old pick-up just bounced through with a little momentum. I did replace the engine at 218k mikes. Again, my fault due to deferred maintenance... and I let it over heat and dumped ice cold water into the motor, cracked the heads. It was cheaper ($1600) to mail order and install a new crate motor from GM myself, than to have the dealer replace the cracked cylinder heads. Would have been even cheaper if I had replaced the original water pump at 200k miles...
So I'm not taking my full size trucks through the king of hammers, the rubicon, or Baja, etc. I ride my dual sport for those adventures. But the burbs and pickup have got us to superb backcountry areas so that we can enjoy our time off.
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To answer the original question, "Why so few GM builds? Look at the pics of my Z71. In those pics, it's loaded with Supplies and shelter for 4-5 ppl for up to 5 days unsupported in the desert and backcountry. There's enough space inside without having to hang baskets with recovery gear etc. I built a simple 2 drawer system for that purpose. The wife and I would like to stop sleeping on the ground in a tent, so we are considering a hardtop RTT or off-road trailer that can double as a dirt bike hauler... not a RV toy hauler.
