To make you feel a bit better, there is very little they could have done to ruin your transmission in such a short time by just changing the oil and filter, especially when you say that the symptoms started IMMEDIATELY after they completed they work.

The most possible cause is that that they may have inadvertently disconnected a vacuum line near the transmission.

Another possible cause is fluid related, and there are three possibilities:

1) Substantially Over-filled
2) Substantially Under-filled
3) Wrong Fluid.

I would bet on a Vacuum line being disconnected, but in either case I doubt that any permanent damage would have been caused.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]