Hi mechies

I've been having trouble with the rear derailleur on my commuter mtb, but I'm not sure what's wrong. It skips and gets stuck and does a lot of weird stuff, and there is quite a bit of sideways play compared to the frame. I took it off today to look closer at it, and the jockey wheels are worn but not exceedingly so, but are pretty wobbly side-to-side. So I'll probably be changing those, which should help shifting precision. The derailleur itself seemed good, tight spring and smooth action, but the mounting bolt was a major hassle to get off and on. It was so stuck that I had to really yank on the allen key to budge it. Is this bolt usually this tight? The sideways play was less once I got it yanked into place again, but it seems to me the derailleur is supposed to be able to pivot around the mounting bolt, apart from the separated plate actually facing the frame. At the moment I don't think anything can pivot around the bolt without a lot of force.

Should I take it off again and disassemble it, so that I can grease up whatever is stuck? Or isn't this really a pivot point in normal action? I'm a little leery of disassembling it since I know there are springs and stuff inside. This is a Shimano Deore der, by the way.