The vibration at speed is concerning , could have a bit of crabbing going on.
And trying to re-straighten could make things worse.
If you have a flat level area, or go to a parking lot, lol, you can use a plum bob and tape to mark spots on the ground from various spots on the frame, then move truck and measure distances.
Bit of a "how-to" here: https://www.vintagepowerwagons.com/p...ts/15frame.pdf
Bumper could be new and frame already tweaked back via come along enough to sell it but not actually repaired.
Look around for other signs of possible repaired damage.
Frame straightening cost is based on time it takes, and that is based on how many directions frame has to be pulled and what disassembly is needed to do it.
It isn't "cheap" but really depends on value of vehicle, which may be why you got a good deal on this one.
4-wheel alignment is not too expensive, and that might tell you if frame is bent too much.