Something strange happen to my car last week. When the transmission switches gears i hear some loud knock from the back of the car. It happens also when i put the gear from park into drive or from drive to reverse.
I don't have any clue what it could be.

And there is something more.

When i start the car he runs good and smooth, but as soon as i put little throttle the car shakes like crazy. It shakes the most when i hold the throttle at 1.000 rpm. if i rev the car up to 2.000rpm it doesn't shakes so much.

Please i need some good and serious explanation for this problem.
I have money so i want to fix it as fast as i can.

I solved the knocking on the rear by changing the dogbones. I used a mechanic stethoscope, let someone push the trunk down and when it moved I checked the noise with the stethoscope.
http://twrite.org/shogunnew/fixes/subframe.html#Replace_the_dogbones:
But such a noise can come from many things. That is just one of the possibilities.
Could be a joint of the driveshaft, the driveline, you also just say rear, where? Or is it more under the car between the seats? Then it is the center bearing.
Best is you drive the car on a pit or on a carlift and check it from below by turning the wheels, remocing the heat shield from driveline etc.

When i start the car he runs good and smooth, but as soon as i put little throttle the car shakes like crazy. It shakes the most when i hold the throttle at 1.000 rpm. if i rev the car up to 2.000rpm it doesn't shakes so much.

let the engine run in idle, use brake cleaner and spray it around the intakes, NOT directly into the airfilters of course. Pull the intakes up and down and spray to see if there is faulse air entering the intake system. Check for visible cracks in rubber parts in intake, check the hose clamps.

Ask a shop to make a DIS diagnosis.
Maybe you get some fault codes. When have you changed the distributors and distributor rotors? Check them. Do they have haircracks? Inside?
You can also measure the ignition wires with an Ohmmeter, as well as the plug connectors.
Also the distributors.
Buy A Bentley repair Manual
http://www.bimmerzone.com/bmwproducts.htm
You can also buy a Peake fault code tool, but on a 1989 750 it will not show mahy faults, on a 1993 much more.
My grey 750 is also a 1989 model, same like yours.
My car is 11/1988, so I use chart 15, all I could read is according to the book (if there would be such faults):

1 DME control unit selftest
3 electric fuel pump relay/TR signal
5 evaporative purge control valve
7 air flow meter
OA emission (lambda) control
OF check engine lamp
10 fuel injectors cyl. 1,3,5 or 7,9,11
11 fuel injectors cyl. 2,4,6 or 8,10,12
17 oxy sensor heating relay
1C oxy sensor
25 control unit supply
2b idle CO potentiometer
2c intake air temperature sensor
2d coolant temperature sensor
33 ignition angle
36 torque convertor clutch
64 unspecified DME output stage
------------------
a modern BMW repair shop with a DIS can read much more and in depth.
So first you may visit BMW to make a fault diagnosis.

mine shakes in park when i rev 1k and up. also while changing my plugs one was drouned in fuel so obviousy no spark.i went to replace the one cable and ofcourse it has to be the one with some kind of sensor or coil wraped around it. i have no idea what it is for(auto timing adjustment?).and went to pick up a set of bosh wires, but none had this thing on it

That are the cylinder identification sensors and you need 2 of them!
http://bmwe32.masscom.net/sean750/Replacing_sparkplugWiringV12/ReplacingSparkPlugWireSet_V12.htm

And they can be bought separately, and when you install them, make sure they are fitted correct, because the Bentley is wrong where the plugs go!
http://bmwe32.masscom.net/sean750/overview/OverviewServiceManuals_versie12.htm

read this thread
http://www.bimmerboard.com/forums/posts/659518
The sensors on ignition leads 6 and 12 are not there to disrupt the injector duration, they are there to identify the camshaft position and are a direct forerunner of the camshaft sensor. The sensors are called 'cylinder identification sensors' and detect when cylinders 6 or 12 fire, they do not measure the primary circuit, how could they, they are stuck on the ignition leads! This is not part of the EML system, any failure will not light the EML indicator. It is a basic ECU input that is found on straight-6's that do not have EML as well as EML variants of both the M30 and M70.

Without the cylinder identification sensors, the ECU's do not know which of the two unique revolutions the crankshaft is on. This does not matter to the ignition as the spark is directed to the correct cylinder by the distributor. It does matter to the injection circuits which should fire every two crankshaft revolutions or every one camshaft revolution.

During cranking the only reference signal is the crankshaft sensor and 'double injection' is maintained until the ignition from cylinders 6 and 12 are detected. From this point on the ECU is aware of the correct camshaft position and can fire the injectors every two crankshaft revolutions. If the cylinder identification sensors fail to detect ignition pulses then the injectors continue to fire every crankshaft revolution but the injector duration is reduced to prevent damage to the catalytic converters.

if you need a used set of wires for the 750, AKnepper has one for sale as he sold his 750 as he started to go to University
http://www.bimmerboard.com/forums/posts/664574

Thanks a lot Shogun.