1980 300d shifting pattern

[Lefty]

I am new to the diesel world, (had a '58 180A back awhile) having been lucky enough to purchase a pristeen '80 300d with little rust (only on the bottom of the doors) and a lovely interior.
My question, which will undoubtedly seem rather pedestrian to all you experts out there, is as follows: after having done all the obvious filter changes, and a purge, I have found the shifting pattern in "D" to be of little use and have been directly shifting using the lever. I start in "S"(which sets off in "L") that automatic shift occurs at about 24 mph, and I then shift manually into "D" at forty five or so, using the sound/stress of the engine to pick the moment (have lots of experience with standard trannys)...so, two things...am I hurting the transmission by doing this? (I know about the "kickdown" switch, which I replaced and still doesn't work) ...and further. I cannot feel the final shift into fourth gear, even when driving at 75-80 mph. I know this baby has four gears, is that final shift supposed to occur at the marks on the speedometer., 55 mph?
All info is appreciated...