Try out Toyotamotorhome.org
This is something that comes up often enough.
The thing to understand is that there aren't really "Sunrader frames". There are trucks. On those trucks, RV manufacturers attached their fiberglass
campers, with some modifications.
The modifications typically include a frame extension to the truck, and some beefier suspension.
You will find no truck that you can just plop a Sunrader body down onto. They will all require a frame extension. Are you talking 17' or 21'? Either way you'll need a frame extension, but obviously a much longer one for the 21'.
But the Sunrader is just a fiberglass
shell. It has no frame. You take the truck, modify it to be long enough and strong enough to handle the camper, then set the camper down on it.
That will be at least as big a job as converting a 2wd to a 4wd. It partially depends on what your skill set is.
Taking a 2wd Sunrader and making it 4wd is a lot of work. But probably not as much work as taking a Sunrader body off a 2wd truck, then modifying a 4wd truck and putting the body on it. People have done both but I think for whatever reason it's more common to take the Sunrader off the 2wd truck and put it on a 4x4. Probably because that way you can use a more modern truck, rather than being stuck with whatever year 2wd the Sunrader happened to come on.
Just be realistic about cost. If you spend $8,000 on a 2wd Sunrader, then buy a 4x4 truck, then all the materials, time, thought and energy...at what point does a $20K 4x4 Sunrader, ready to camp with, start sounding like the better option?