When I decelerate, the tachometer needle goes crazy, jumping all over the place. What is the solution?Do I have to replace the whole instrument cluster?

You have to remove the instrument panel and rotate it back in to position.

Sounds like the sending unit has an intermittent short to ground. Unhook the the wire from the sending unit, and see if the needle stops jumping around. If it quits jumping around, replace the sending unit. If it still jumps around, you have a bare spot somewhere on the wire and it is hitting ground which will cause the needle to jump. Cut out the bad part of the wire and splice in a new section. That should fix it.

instrument cluster is bad

Sounds like a possible speed sensor problem involving the wiring harness or connections. As the car decelerates it bends the wires and may cause a communication problem between the engine, transmission, and the gauges. Or, it's the gauge itself.

COULD BE GROUND PROBLEMS OR THE TACH GOING BAD.EVEN THOUGH CAR SEEMS TO BE RUNNING GOOD IDLING GOOD.SOME TIMES A SPARK PLUG MISFIRE.CAUSING A LITTLE ROUGH IDLING CAUSING TACH NEEDLE TO GO CRAZY.I SEEN ALTERNATOR WITH VOLTAGE REGULATOR GOING BAD CAUSE A POWER SURGE CAUSING HEAD LIGHTS TO DIM A LITTLE THEN BRIGHT WHILE ENGINE IDLING.THIS WILL CAUSE TACH METER NEEDLE BOUNCE AROUND ALSO.

On the 05 Mustang instrument clusters this is a common problem, it's caused by failing stepper motors on the cluster's circuit board. It can be fixed though, check out this page for more on how to get it repaired - 05 mustang instrument cluster repair

it is not a connection problem. The instrument panel will need to be replace and reprogrammed.

possible ignition coil itself because tachometer takes signal from ignition coil.you may also have check engine light on too.

Purchase a new keyless remote FOB. The signal from the remote is weak and loses contact with the anti-theft module in the instrument cluster. We have 2 sets of keys and 1 worked while the other caused the instrument cluster needles to jump and jam the gear selector. The problem is not the car's battery, alternator, wiring or instrument cluster.

Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Silverado instrument cluster repair
Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Trailblazer instrument cluster repair