w126 doesn't stay in park
 

I am trying to figure out what exactly the cause of this problem is. Whenever I want to actually get the car into park, I have to wiggle the shifter around relentlessly until I hear it click into park. Whenever I don't bother to get it into park and just use the parking brake, the car will not start until I throw it into neutral. However, it never fails to start in neutral. Would the problem be the shift linkage bushing, shift lever bushing, neutral safety switch, or something else, like the need for a simple readjustment?

Most likely the two shift linkage bushings under the car are disintegrated and in need of replacement.

on my 83 sd, When I tried to adjust the shifter housing so that it matched up perfectly with the wood grain, I lost my park too. I put it back the way I found it. If it is disintegrated, your parking pawl won't engage. Put it up on ramps and look at the shifter bushing.

Other than the condition of the linkage, how are your engine and transmission mounts? If the trans is moving around the linkage won't work right.

If your shifter is loose and you have to wiggle it a lot, then it's almost certainly your linkage bushings. They're cheap. Somewhat of a pain to install. Some folks press them into place with a C-clamp, but I find it is much easier and just as effective to simply cut the little buggers once with a razor (turning an O into a C but not removing any material) and then "screwing" them into place.

You will have to take off the shift lever from the trans housing to make it easier, and that means that you will probably knock the car out of park whilst underneath it so.... PARKING BRAKE and CHOCKS mandatory!

Update: Replaced the shift linkage bushings- problem solved!

FYI
 

Here is further data on what he repaired.

Broken Neutral Safety Switch and replacing shifter bushings
http://www.peachparts.com/shopforum/diesel-discussion/96048-broken-neutral-safety-switch-replacing-shifter-bushings.html#post646647

Replacing all shifter bushings on W126 - p.1
http://www.peachparts.com/shopforum/diesel-discussion/112582-replacing-all-shifter-bushings-w126-p-1-a.html