I bought a 2000 off ebay that I regret. Really nice inside, and ok mileage at 182K. Got it home and found a rust hole in the frame big enough to stick your thumb in. Right next to the cross-member. I won't sell it to anyone. My ex batted her eyes and got it to pass inspection so it is registered on the road, but I won't take it off road. I went through the same thought process you did re replacing the frame, and visited a couple of respected LC specialist shops in Northern Virginia. The telling quote was when I asked the hypothetical: "If I said money was no object, what would it cost to replace the frame and do this up right." The response was: we would start by taking your money and buying you another truck. I did confirm the thing is not actually unsafe to drive around on-road, but I am expecting a failure one day soon.
Also on rust, it was slightly pulling to one side, but still technically in-spec. Sears (which I figure can handle an alignment) tried like anything to get the bushings off which needed to be replaced. No go. So as others have said, it is not just the frame, but everything else tends to be frozen in place.
Just for fun here is a photo to my eternal shame for having purchased:
Nice, right?
Still looks great, right? Wait a sec....what's that dark spot on the cross-member!? (Which I missed on ebay.) Its a hole. And not the only one.
I have a 1998 coming in from Dubai that has 347K miles on it but is in great mechanical shape (I've had it gone through pretty thoroughly.) This 2000 is going to serve as the donor for the interior, and maybe a few other parts. Then this sad truck will be donated to the local public radio station. I won't sell it to anyone.