Asking for a new work truck that for my back

Hello, I have a job that involves a lot of driving. I'd say at least 50% of my day is spent driving too and from jobs with my work truck.
I've been doing this job for two years now with this particular truck.

Beyond the numerous breakdowns and known issues with this truck that the company would rather slap a bandaid on than properly fix (or just replace the truck), the seat in this truck is... it's so bad. There's no support, it's too low to the ground so my knees prop up above my waist line without adding towels and pillows, it's got practically no cushion on the back or bottom.

The company I work for has made no visible efforts to get me a new truck. I drive the smallest, oldest, most break-down prone truck in the branch. I guess that's irrelevant but lord is it frustrating.

Like I mentioned, been doing this two years with this truck. About 6 months in I developed hip pain in my left hip, feels like where the legs meets into the hip (idk anatomy, but it hurts). It's constantly sore now, i feel like getting home from driving all day is just a game of tending to my sore hip and taking it easy. It's coming to the point where, I wouldn't say my left leg feels like it's going numb, but I can feel this weird pressure going down my leg that's concerning.

NOT ASKING FOR MEDICAL ADVICE. I am seeing a doctor in June.

However, if this is an issue related to my work truck (wouldn't be surprised), what actions should I be taking RIGHT NOW to properly address this if this does turn into some sort of work-related injury? Of course, I'll follow all the directions and advice from my doctor and go from there, so this is gonna take a while with how medical stuff works in the US, but I want to know what to say to my boss of the branch to get it onto the records that I need something new. Should I contact HR about this?

Never faced anything like this, I don't have anyone to turn to, any advice or experience you can share is appreciated. Happy Spring, everyone.