Rusted frame? Next step?

I'd look those diff housings over carefully. I have a set of 40k lb Rockwells that are rusted through.

 

Looking from the rear of the truck toward the fifth wheel plate, see if the top flange on frame is nice and level. If you see any warping that is a sign of rust in there.

 

Anytime. Here are pics of when I took off the fifth wheel plate and cleaned. You can see the crap that formed on top of the frame rail and how it ate away at it. If you see that you need to get it stopped.

 

that’s nothing

 

Got quoted 5k to sandblast and paint a frame,still shopping around

 

Watch the frame, could be twisted /bent, cracked , check the height every end from ground to bottom of frame on leveled flat shop floor;look from behind along the frame for waves, and go under the cab, check the cross members from snapped missing huck bolts.
I got rid of my 2008 Volvo truck last year, (still good motor transmission but high mileage), for twisted /bent excessive corrosion frame, was too expensive to replace.
Btw, just for your safety, when you buy a used truck like this remove the hubs check the bearings play and axle spindle , eventually replace bearings if all ok. I got a bearing loose on my spindle chewing the axle at ~1 mil miles, luckily i replaced axle in time; don't ignore abs/tcs light.