I'd want to make sure it needs opened up before I started looking for a rebuild. BUT, if you description is corrrect. So this is why it's critical when explaing a problem that you are very very specific on what happens.
So I see we now know we are talking about a 700r4, so we can leave vac modulator out of the issue since 700r4 doesn't have one like a Th350 or Th400 does. So, we're down to TV cable and Governor.
You first mentioned it just won't shift out of first gear. That could be a hung up TV valve which cable could be or not be what's causing the TV valve to hang up. Orrr, the governer either has a stripped gear or the valve is stuck in the governor.
Then you mention, it doesn't shift out of first gear, but it takes off in first, then the RPMS go way up but the truck won't go. That tells me, it is actually shift from 1st gear, but it's trying to get to 2nd gear and 2nd gear doesn't exist so the RPMS runaway since it's hunting for something that no longer exists which would mean it needs to come out and be rebuilt. I have a feeling if you took the pan off, you'll like to find some shiney mangled metal parts. I think the intermediate roller cluch broke.
I do know a guy who once drove a truck with no second gear for over a year before he rebuilt it. There's a trick to it though. So you put in manual 1st, take off, pick up some speed, wind it up pretty good and 700r4 having a lower 1st gear, it might be tough to get the speed fast enough, but what you're doing is trying to get enough speed in manual first gear, then let off the throttle completely and put it in D, not OD, but D as in 3rd gear and it might go to 3rd then OD if you're on the freeway going fast enough. By doing this, you're faking out the governor so it'll skip 2nd and go into 3rd. I know it works on a Th350, maybe not a 700r4. But they 2 do have a few similaries, but they also have several differences.