with a 3.07 diff you should see approx:
SPEED RPM
60 2750
65 3000
75 3400
80 3650
85 3850
90 4100

I read this with my 300D and 3.07 gears before I installed a 2.88 diff. Rounded as close as I can read.
If your tach is correct (and I would think it hard to be off by 1000 RPM!) then you just may either be stuck in 3rd gear or the diff gears are not what is stamped on the case. Like Brian says, its time to manually count the revs (up on a hoist I presume)

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!