What would cause my 2000 ranger to buck between 45 and 50 mph going up hill

Does it do the bucking when you use the cruise control? If so, it's probably a bad TPS (Throttle Position Sensor)!

When was the last time you changed the fuel filter. My Ford has same problem an it was a fuel filter.

plugged transmission filter maybe not letting it downshift

The engine may misfire and/or buck while driving at freeway speeds with no fault code stored in the powertrain control module (PCM). This can be caused by an intermittent fault with the camshaft synchronizer assembly. This condition can be diagnosed by monitoring knock sensor data as per Ford service bulletin #05-22-12. Replacement of the synchronizer assembly will be required to correct this concern.
https://repairpal.com/engine-buckmisfire-at-freeway-speeds-934

I hope this helps

Get that sway bar replaced!!! Yes it will be one of the reasons for the noise. Because it's broken it's pushing the whole suspension out of shape. Get it sorted out before it starts costing you BIG bucks.

could be sensor has failed or could be in need of tuneup...

You might need some programming. Could be timing. Timing would cause a problem. I think your cam and crank need to work together, if one was changed, and other wasnt, could have some type of imbalance.

something that old your first goal is to locate a serviceable used unit, the whole cluster not just the speedo, via eBay or a junkyard. less than 50 bucks, but there is no warranty.
once located you look for a video on youtube of someone pulling the cluster, or buy a repair manual for a few bucks on eBay. watch someone do it or follow the instructions.
you may want to try that in reverse as the thing is only worth 500 bucks and will not be worth another cent with a working speedo. if it is too much work for you, then most likely you will not want a used speedo laying around.

could possibly be your head gasket or even a cracked head

I had a 1990 cadillac deville with a bad computer that wouldn't deliver fuel to one of the cylinder's injector and this caused it to buck at 35 mph.