I've never been very good at tremolo picking and that's starting to bite me
know that I've started playing metal.
All Nightmare Long by Metallica right around 0:37
Divinations by Mastodon 2:13 - 2:55 (Referring to the rhythm guitar)
Any advice?
Well, practice picking with controlled articulate wrist movement, tension-free. Use the stickies from this forum, watch Freepower's videos, and practice with patience. It takes time. Things like pick angle and picking with only the tip of the pick can be of help.

If you want more specific advice/critique, upload of a video of you playing and post it in the technique analysis sticky.