Just started playing the game, and it's fun, so I'm not looking for any plot or quest spoilers.

I've found a couple of tubes of HealGrease that are tucked in out of the way places. I can walk up to them but they're not on my exact level. For example, one is sitting on top of a bulldozer. I can walk right up to it so it looks like it's only a couple of feet over my head, but I can't seem to take it, and I can't see any way to climb up on top of the bulldozer.

I assume I'm missing some obvious way to get these items,

Nope. You're not missing anything.
Years ago when I first played this game I must have spent an hour trying to get that damn healgrease. Fond memories. Anyway I finally gave up.
Without spoiling anything (I hope) you will get a party member later who will have a world skill that will allow you to go back to places like this and grab the unreachable stuff. You just can't get it early in the game.

Thanks. I figured it must be something like that. Or a taller party member.

It's strange playing a 3rd-person-perspective game like this and not being able to jump or climb.

Pretty sure you'll meet somebody with a tractor beam worldskill at some point. I bet that would be useful.