I did a search, but didn't find my exact problem.

Background:
I've had my 83 300SD for 3 months, running homebrew BD the whole time. Removed the ALDA about 2 months ago, capped the ALDA vac line.

Last night, doing about 40mph, noticed revs were high. Realized it never shifted into 4th. Feathered the throttle to get it to shift..it didn't. Put it in "3" and back to "D", still no shift. I read other posts that it wasn't good to go to neutral and back to "D" while driving, but I thought at 40 and no load that I wouldn't damage anything. I did it, and it subsequently found 4th and was fine after that, until the next time I came to a stop. Same problem on the way to work this morning...no 4th unless I go to neutral and back.

The 1-2 shift and 2-3 shift are fine, (although they may be occurring at slightly higher rpm, it hard to tell). No other driveability issues.

I need to do 150 highway miles tonight, and I'd like to do it with a 4th gear if possible. Anybody have a quick solution that I can do on my lunch hour?

Thanks

Mark in MA