I don't suppose you have the shop manual and I am sure one of the very knowledgeable folks hear will be able to answer your post with expert ease, but for my $0.02 worth regarding theory of operation; Automatic transmissions have clutches and bands that control which gears and gear-rings are engaged and which are held stationary through the various ranges the transmission shifts through. On many tannys the bands are adjustable which means you can reduce the clearance between the band and the gear-ring it attempts to keep from spinning in a certain gear range.
Here are some pictures of various bands One side is hydraulically pressed to tighten against the smooth outer side of the gear-ring while the other side usually has a stationary, automatic, or manually adjustable rest-stop. In the case of the former two you must replace the band if is determined that it is slipping, but in the case of the manually adjustable band you usually adjust by backing off a locking nut and screwing in a bolt and then backing off a prescribed amount and then tightening the locking nut. I did this on my Dodge a few weeks ago and the "rear" band required me to drop the pan to gain access to the adjustment while the "front" band adjustment was external. However you must first find a shift or range chart for your transmission. That will tell you which combination of clutches and bands are engaged for each gear and consequently let you determine which if any bands are your problem; Clutches unfortunately are not serviceable as far as I know. Also I have heard some MB trannys start off in second gear range unless you floor the accelerator, so just be careful you are diagnosing the correct gear range. Study the chart and try testing in R,1,2 & D separately; be aware that shifter range 2 may not be the same as second gear in D as was my case. But if it is something else like the shift valve body, you'll need someone else's advice. Good luck! -Lou