my '85 300sd will go into reverse but no forward gears. was starting off and soft boom and started rolling back down the hill reverse works fine though. all foward gears are out.

 

There is no telling for sure till you investigate but, you have the symptoms of a Broken B2 Piston or B2 Brake Band in the Transmission.

I had to replace mine and on the 300D there is less room to do that then on your Chassis.

I did not order the Parts needed to replace the B2 Piston till I had verfied that the B2 Piston was broke and could poked on the Brake Band to get some idea if it was broken or not.

I don't know if that is the only cause of your Symptoms but the B2 Piston is accessed from a cover on the outside of the Transmission making it somewhat easy to inspect as you don't need to fool with other innards of the Transmission to get at it.

 

If it turns out to be the B2 Piston this is the Parts List.

B2 Piston and parts:

B2 piston number used to order 107 270 04 32 (1702700432)

Outer Cover O-ring 005 997 70 48

plastic (an upgrade that replaces the Aluminum) bushing 126 277 08 50

Lip Seal 006 997 73 47