1992 Plymouth Acclaim Shakes harshly

Shakes or Wobbles problem
1992 Plymouth Acclaim 6 cyl Front Wheel Drive Automatic? Miles

My car seems to run great once I hit about 30 mph. It has been running fine until just recently. Now when I start the car, it doesnt sound quite right. Maybe missing, not sure. When I shift into drive or reverse, the motor sounds entirely different, like it wants to stall. When I take my foot off the brake, the car shakes roughly. When I apply gas in drive, it usually straightens out pretty quick until I let off the gas again, but doesn't shake harshly again until I slow down to below 5 mph. When in reverse, the car does better when I do not apply gas, when I do apply gas in reverse, it shakes harder than it does when I don't apply gas in drive. I just changed my air filter, and I will be changing my spark plugs and wires later this week (need tools). What could be some other causes for this? The car has not stalled as of yet. Thanks for your hep!