You mention Forest Service roads and sounds like you plan to be in North America? so I'm not clear why you want an older mechanical truck.
But on to your question. The answer is it all depends. Every vehicle has limitations. if it gets muddy and you have 2WD (well or 4WD) and you just cannot go you need options - Wait a day or two for the mud to clear, use a winch, MaxTrax, branches (please remove them when done), or go another way. You will always find a track where it is too muddy to travel - with 2WD that will just happen sooner - but you have options (just mentioned) that you will need whether you are 2WD or 4WD. And if you think it cannot be done read about Dot Becker driving her RWD Ford van from Europe to Zimbabwe down the west coast of Africa. Of course, Dot is one of the best off road drivers I've ever seen.
will probably only venture in the US & Canada, possibly Mexico but that is the limit
i want an older mechanical truck because it is what i am familiar with and comfortable wrenching on not to mention the historical longevity
have owned mechanical diesels for 25 years and favor reliability and simplicity of the engine bay and cab console with relays & cable actuated levers over computer assisted anything (car related)
options is what i am considering and appreciate the reminder that time is also an option. conditions can improve if one merely wait