My speedometer quit working. It's pegged @ 120 all the time. It's a 2004 HD with the Duramax. Any ideas on how to fix it?

The tiny electric motors that move the needles on the cluster are prone to failing. New sets of them are relartively cheap on E-Bay. Takes patience and some skill to change them.

Well you know it does not have a speedo cable since you replaced the speed sensor in the transmission.
My guess would be a problem with the speedo or cluster.

the is a problem with the servos in the dash doard i have a 2003 duramax i found on ebay some one to relplace all the servos wih up dated ones and put it back in its the cheepest and you can drive with it out use your gps for mph the dealer will fix it just more time and money

look and see if the needle jumped passed the little pin on the face you might need to remove the lens and pop it back over the pin

If you experiencing issue with gauges not resetting, for example, speedometer shows speed above 120 mph while car standing still, it's quite common problem in 2004 Tahoes. You have few options:
- go to dealership and replace the cluster (a lot of money)
- search internet and find a shop to repair cluster. Since the problem is quite common, there are many options. Will cost much less then dealership
- if you are comfortable with soldering gun - buy replacement motors on Amazon, disassemble dashboard, remove cluster, replace motors. This is the cheapest option - about $35 and hour and a half of your time to replace all motors in cluster.
- last option is for people who don't like spending money (and fixing issue too), put car on emergency break, shift in gear, turn key to start engine. With every attempt gauges will reset by a bit. Repeat until gauges go all the way to zero.

they have a extnded warranty on the speedometer cluster 7 years or 70,000 miles

when the sensor was installed it was installed wrong.
They wired it into the turn signal feed.
have your dealer or, an auto electrician fix the wiring to your sensor.

sometimes you can play with the ign key and turning it off and on a few times it will go down, I mean on, then all the way to lock, like 10 times. you will need the cluster replaced. any good auto shop can get a rebuilt unit or send yours off for repair.

The problem is MOST LIKELY a bad solder connection on the instrument cluster circuit board.

replace speed sensor