Good morning Felix, sorry for delay, my name is ***** ***** you for asking your question, I will try and help you out.
Since the speedometer is reading erratically that will cause your RPM to be limited to 1500 so your issue is between VSS and ECM,
since you replaced VSS on back of trans make sure trans yoke is tight, remove speed sensor and see if you can spin tone ring with a screwdriver, if you can yoke is loose or tone ring is cracked, if that all checks out you need to replace wiring harness for VSS to ECM.
Also make sure VSS is set correctly, turn all the way in and then back off half turn and lock down locknut.
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