Engine shakes when in drive

Check your engine mounts. It's an easy check. With the hood up, have a friend jump in the car and crank the engine. Look for the engine to "shake" or "jump" when he shifts the vehicle from park to reverse and back to park again.

what about the spark plugs HT Leads ?

start vehicle and place in drive while holding brake, depress accelerator and watch engine for lift, place in reverse and repeat.

The spark plugs are under the coil packs. remove the top plastic engine cover and you will see a coil for each cylinder, remove one coil at a time and with coil removed you will see spark plug in hole.

how long did you drive it after putting this additive in and did you add any fresh fuel, is your engine light on? because and electrical issues will trigger your engine light, answer these questions and someone may be able to help

The check engine light is telling you the computer is detecting a problem from one or several sensors. You will need to have someone use a scanner to pull the codes. Places like auto zone offer this service for free(they want to sell you parts). Once you find out the codes then you can try and determine what the problem is. Could be anything from a bad plug wire to a plugged fuel filter.

Idle sounds like it's too low. This is controlled by the computer. Is the check engine light on?
Make sure your tune up is up to date.

check the level in your gear box.

go to any kragen or autozone parts store, they will run the test for trouble code that turned light on for free, tell them u want an OBD 2 trouble code test, then post the code here.

I assume that the 7.5 amp fuse blew again, yes? If so, it looks like you may have a locked up A/C compresser, or too much freon in the system (if somebody tried to add some to make the A/C come on). If the compressor is locking up, the engine will turn slow, at least until the drive belt or the fuse fails.