I had the same problem on my '82 300SD. After attempting to loosen the #3 phillips screws holding the hindges, with absolutely no success, I got out the WD-40; and, liberally squirted all of the hardware, including the latching mechanism. The WS-40 alone fixed the problem. It still closes a little harder than the passenger door; but, nevertheless, closes with just a fairly normal shove. I used to have to slam it to latch. It turns out that the resistance was in the latching mechanism, not in missalignment.

Jeff Naumann