Ahhh, a topic dear to my heart ! I cut my teeth offroading with 2wd p/u trucks before getting my first actual 4x4 in 1988 .
For historical context 4wd trucks didn't become widespread until late '80s - early '90s . Prior to this the norm for hunting, fishing, farmers, construction guys, etc was 2wd pickups ( obviously full size American trucks in that era ) with snow/ mud tires , and preferably posi .
Extreme rockcrawling and deep mud bogs are somthing else , but those are things people usually do on purpose . For most working or outdoor recreation decent ground clearence and good tires would handle up to medium rocky trails, and medium muddy . Certainly any USFS " road " , and many USFS " trails " . My '72 F-100 w/ 11-15 Power Cats on back could negotiate snow at least until it got deeper than the frame .
The biggest difference with 2wd off roading is knowing what you're doing . Reading terrian . Picking your line carefully . Making use of momentum .
4x4 lets you be inattentive, and lackadaisical, and have the extra drive wheels carry you through anyway . And in slippery conditions , 4x4 gives more steering control from conventional steering wheel use, where 2wd needs to understand momentum , and judicious use of gas and brakes to steer the truck .
Bottom line - A knowledgeable driver can go 98% of the places , as could a 4x4 of equal ground clearence .
For an off road 2wd , go one step gnarly'er for drive tires than you would for a similar 4x4 . An at least minimal self recovery like a come along is recomended for 4x4 , and would be even more prudent for 2wd .