I think that the B2 piston on my 300SD is on the way out.
 

... how much time do I have left before it's shot?

Occasionally when I stomp on the pedal from a stop or a crawl, the engine revs and the car clunks into second gear. From what I've read, that's probably the B2 piston going out. If I drive the car carefully, there are no issues. Can I baby this thing along for a while? Is there enough room in the transmission tunnel that I won't have to remove the drive shaft and lower the transmission to get access to the B2 piston? How much more difficult is it than changing the vacuum modulator? Any guess on how much a shop would charge to do the job? Thanks

You got a few days but its not worth the tow if you get stuck ,its failed dont risk it would be the right way to say this.Once forward movement does fail a working reverse will need to be identified .Trannie shop with questions ,forum we are guessing for the most part .Most shops youll find they dont like to fix just a small problem ,theyll alawys suggest a rebuilt trannie ,key is pushing warranties, at the end your looking at $2600. to do the job .Theirs Alot of diyers here at Peachparts that have done the b2 piston ,its doable but dangerous if you dont have a lift. Me ,same problem same delimia.

Here are some writeups. Check them out to see if you are comfortable with the job.

b2piston
How to Replace Your Oil Cooler Lines

I would stop driving the car. It's not worth getting stranded somewhere. The B2 piston is near the rear of the transmission, so you will need to drop it down. You will have an easier time if you jack the car up and support it on all 4 sides.

Read some more articles on the B2 piston and band, there should be plenty of info on it here. Be prepared for replacing the band as well. Often times it is not the piston that is the source of the problem. These transmissions are getting old.