Due to lack of oil in my 1999 navigator...it began to shake and tremble as if it was going to turn off i have added oil n it is still shaking and trembling just not as bad..can you help me out

check the dipstick but I suspect that with 6 quarts ( gallon and a half)in an already filled sump that takes 4 quarts max is trouble in itself
coming to a stop and shaking indicates a vacuum leak or problem injectors
take it to an accredited service center , have the oil level adjusted and have the fault codes read

Why Does My Car Wobble When I Brake?

www.popularmechanics.com/cars/.../why-does-my-car-wobble-when-i-b...

If your brake pads are still in fine shape but the car trembles when you breaknonetheless, this could be your problem. ... Lately when I hit the brakes, the car trembles. ... In my experience if the steering wheel shakes or the front suspensionshudders when on the brakes, the front ...

Brake Vibration: Common Questions Answered - EBC Brakes

ebcbrakes.com/articles/brake-vibration-common-questions-answered/

Brake vibration is where a shaking motion occurs when the brakes in a car or othervehicle are deployed. This can vary from a slight shaking to a quite severe ..

could be leaking tru head gasket see if motor oil is contaminated/foamy

sounds like you have a cylinder with low compression like a sticking/burnt out valve get a compression check done first then check for air leaks on the inlet manifold ie, the gaskets

Could be coil packs they tend to weak and will misfire

I know it sounds silly but i would be checking oil level to see if its been overfilled. this will cause problems like you have. The engine is probably shuddering because if oil is getting to the air cleaner it is probably in the combustion chambers as well and reducing the sparkplugs ability to perform under load. Quite possible a hose was not reconnected or has come loose giving a vacuume leak which could also cause a problem. Check for these things and see how you go.

If you used one of the 'quick lube' shops, you may not have any oil in it at all. One of them drained the automatic transmission instead of the oil and even filled the transmission with engine oil, destroying it. $6000 later with a law suit behind her, she got her car and money back. Most of those shops employ idiots and I would never entrust any vehicle to them.

I don't know how much oil you had to put in but it sounds like you have maybe more than opne issue and both sound major. One is engine reving and speed not picking up sounds like trans problem. Two the engine shaking and making noise and being run low on oil leans toward possible engine damage. If your mechanicly inclined check all fluids and when checking trans fluid smell it. If it smells burnt the trans has been damaged. Check oil pressure and make sure coolant is full.You may have to take it into the shop.

Is it idling too fast? More the 850 rpm?

need to have your rotors turned