Could be a lot of things but it being driven by load and when at lower RPM is nice as you can replicate the issue. Here are some places to start (also in order of cheapest to do first):
1. Dirty Throttle Body - Clean with some alcohol and a rag.
2. MAF sensor dirty - Clean with some MAF spray
3. Vacuum Leak - find and replace cracked hoses
4. Dirty Air Filter - replace
5. Dirty Injectors - try some injector cleaner in a few tanks of gas (if this improves idle, consider sending them in to be professionally cleaned/metered).
6. Spark Plugs - if old or fouled, replace. If fouled, start looking for reason why. Check to make sure none of them have backed out over time (they can get loose and you will get blow-by)
7. Coil Packs - inspect packs, boots, etc. If they are cracked or discolored, replace. If boots are old and brittle, get reboot kit. Buy a spare to keep in vehicle.
8. An accessory on motor might be going out or have a bad bearing
a. take of serp belt. Spin all pulleys - listen/look for wobbling and noise.
b. check fan assembly for any wobbling, check alt/ac compressor, etc. for bad bearings.
I'd report back if none of that seems to help. These motors are incredibly smooth, to the point that even old motor mounts aren't going to transmit a lot of vibration from them. Not to say they don't get old and dry out, but they don't fail - most folks here are on original trans/motor mounts. While they'll make things even smoother, they aren't going to make a big difference like you're talking about.
