Tach not working on 06 ram 1500 5.7

You probably should take it to a qualified repair shop before you replace part's that you have no clue how to diagnose . Replacing a PCM/ECM would need programming , dealer only .

sensor maybe

it is a indicator of low rpm or too high rpm most engine runs on idle around 1,000 1,200 rpms

I put aftermarket tail light and headlights LED in my in my truck 06 Dodge Ram 1500 turn signals got hyper flash trying to replace the relay cannot find

Around 750 rpm in Neutral.

run the fault codes
the rpms change indicates a ratio change up then back down indicating a TR or VSS problem

The cable running to the back of the tach is probably broke. The only way to fix it is to take the dash apart so that you can get to the tach. unhook the cable, gently push it through the firewall as far as you can, then pop the hood, find your tach cable and gently pull it the rest of the way through the firewall, disconnect it from the motor and put on a new cable, reversing everything you just did.

Tach spring is breaking, you can get a seperate tach gauge in place of what you got. or how about this take dash out and get another tach gauge they are removable

change the sender unit on the differential

Well the only real fix for that is to replace IPC. The new one should come programmed and ready to go