I have 1981 Mb 300sd,when I start moving uphill direction the transmision feel like slip out.Does the tranmision needs to be rebuilt?

The car is idling too slow. There should be an adjustment to increase the RPM. Have someone press the accelerator while you watch what moves (while it is not running) You will probably find an adjustment screw to make it idle a little faster.

should be on the steering shaft under the dash

It is located on the side of the transmission built into the neutral safety switch and unless you have worked under a car correctly before, I would advise against it...make sure you dont have any blown fuses or burned out bulbs that would cause the reverse lights to not work...best place I have found for MB info is this forum..registration is free and tech threads a plenty....yours is a W126 body
Title goes here

low on tranmision fluid or transmision problem

oil level incorrect or filter or pump / torque convertor faulty

with the key,its not automatic

You have a failed transmission. You will need ti rebuilt.

sounds like it needs some drained yes... try that as a start and see how you go... good luck

Have you serviced the transmission latley? fluid, filter, band adjustment

Usually under heavy throttle causing the slip is caused by a seal in the pump leaking off hydraulic pressure letting the clutches slip.

Does it slip in all gears or just 1st.?
Does it slip in reverse?
The reason I ask this is because 1st and reverse use different fluid pathways,

also you can take it to a tranny shop and have them check the fluid pressure at the accumlator port to determine what the exact problem is,but my advice is to just drive it easy and make it last as long as you can,a new or rebuild tranny from a dealer would run 2500 or more.

there is a website www.car-part.com that you can use to find a replacement transmission

check sensors on transmision could be bad not picking up engine speed to ingauge 4x4