I have found my P38 quite capable off road. I'd suggest you buy a tirfor in case you get into trouble. For all the mucking around and cost you don't really need a power winch, although I found my old classic went way faster up the hill to my house when I fitted it with a Warn. I did the Monkey Gum Fire Trail in my old classic (prior to fitting the winch and bar) which is a pretty chewed up steep climb at the end and had to get the tirfor out once to help climb a rock. Otherwise it went fine. The P38 goes better than the classic (except downhill as my classic was a manual so had better engine braking).
Its also not so fashionable anymore, but I am a big fan of chains on muddy roads. You would be surprised what you can get through with a set of chains including wet grass on hills. The old man and uncles used to drive on some very ordinary roads with regular old two wheel drive utes with chains if necessary so I guess I am showing off my hillbilly origins.
The best advice I have for off roading is the old land rover motto - as slow as possible, as fast as necessary. And if it gets too ugly, either reverse down the hill and go back or get the tirfor out, if you rely on momentum to get you up ugly bits it is just a matter of time until something breaks.
Carbonado is a dream run, full of history and incredible views. I lead a run up through the snow line a few years ago. Take a break in town for coffee and enjoy the oldest longest operational school in Washington. From there take a look at the old coke ovens lining the street out of town. The fire roads and trails above the town are poorly maintained, but take you to incredible views of the area and sides of Mt Rainier most folks never get to see. Surprise lake? they have a Starbucks and paved roads.... not a real off road destination. Stick to exploring the the fire trails east of Surprise Lake OR head south to the remnants of old Melmont then head east up any trails from there. Either route will give you breath taking views of Rainier and you will have a ball getting there with occasional use of wading height and low range.Iam also thinking about doing a Surprise Lake run near Carbonado,WA this year. Anyone have any experience there in a p38?