THanks for responding to this. I have a 83 240 DL with about 130K on it. This problem started about two months ago, while on a couple thousand mile road trip. It began around the same time that the fuel pumps went out. (but not sure if related)
I've since had both the fuel pumps replaced and this problem is still there. Mechanic says its my rear axle and its shot, could go out at any time. quote 'it could lock up and wheels fall off, very dangerous' Told me to take car to junk jard unless i wanted to spend 2000$ dollars for new axle.
Ok, so, I just spent 800$ or so on new fuel pumps and tires, everything else is running great, so don't want to scrap if don't have to. Hope some of you have some advice on what the problem could be. Hoping its something simple!

Heres the problem: While shifting from 2nd to 3rd gear and while cruising in town in 3rd, if I slow down, feels as if it 'slips' or 'breaks' between gears. I don't think its actually shifting itself, but feels sort of rough, like it wants too. If i am in fourth gear it will do the same. While i am accelerating there is no problem, just when slowing down or cruising in lower gears. Can the fluid just be low in the differental? Something easy like that, or are the tranny mounts, rear axle bearings, ect to blame....
please help, don't want to get rid of this car, but wife won't let me put much more into it...
thanks