Please HELP.

My Mitsubishi Grandis 2.4 Sports version (4 wheel drive) has the following annoying problems I cannot figure out what the problem is:

(a) When travelling at low speeds particularly, it jerks as if mis-firing. I have to release the gas pedal for a second and re-press to get it to stop and I have to repeat this many times and its now a daily thing. We have tried changing plugs, fuel filter and pump but still problem getting worse. HELP

(b) When taking off there is a Thud (Gu) sound at the rear. When reversing and turning at the same time the car has a very painful thud sound and refuses to go or jerks violently with thudding sound at the back as if something is jamming. Reversing in a straight lined is better. HELP