2001 Hyundai Elantra Knocking noise

I don't take care of my car the way I should but I'm a single working mom and never seem to find the time to do everything I need to. I don't have a lot of money so I can't afford to just take my car into a shop and have them look over everything to find the problem and have them tell me a million things are wrong and it's going to cost lots of money to fix it. I'm having a problem when I drive in town on the streets there is no noises or clunking it's just smooth, but when I get on the freeway and start accerating over 50 mph the car starts to shake really bad. The steering wheel shakes the passagenger seat shakes, well the whole car does. If I continue to accelerate to 75-80 the shaking lessens alot, almost so its gone but then when I slow again and speed back up it starts up again. I thought it was the alignment because it pulls to the left just a little bit, but my dad said that the amount of shaking that it is doing can't be just the alignment. I'm wondering if there is something you may think it is so I can just go to a shop and say I need this looked at to see if it needs to get repaired I don't have a lot of money like I said but I need to get it fixed because this is the only car I have and definitely don't have money for a new one. Thanks.