Last week after refueling my tank full, the fuel guage only showed 3/4. After parking the car over night it showed full. Today the fuel guage read way above full (should be 1/2 a tank) I also noticed all the other guages have the indicators/needle all the way to the right. I unplugged the battery and everything went back to normal. Any ideas???

 

Cheapest thing to replace would be all the stepper motors. Common issue with GM vehicles. Parts found on ebay and roughly 10 million youtube videos on how to replace them if you have basic soldering skills.

 

Would one motor going bad cause all the others to malfunction.

 

Normally no.
The malfunction will not effect anything with the truck other than now you wont know how much fuel you have left.

My speedo floats around, but for the most part tells me I am always doing 120mph even when parked.

LoJac is correct. It is probably the steppers
You can get the motors replaced locally probably for 150 bux, or do it yourself for probably 25

 

Some owners have noticed when one motor goes bad they all start to go bad. As I said, it is the cheapest thing to replace. If new motors doesn't fix it then there other routes such as the cluster, modules, etc which are must more costly.