stock tires and rear end but at 50mph By garmin the speedo reads 55. At 60 the speedo reads 70.How can I fix the misreading speedo?

I think you will need a FSM for this.

The way I think it works is the computer counts the pulses off the ABS and uses that to turn a motor to turn the speedo.

Does the odometer also read high? If so the dealer can flash the computer with a different tire size to correct the error. Or you can DIY:

https://appcar-diagfca.com/en/

If the odometer reads correct and the speedo is off, the instrument is faulty.

I'll do a mile marker check and see if the odo is correct. The part that makes me wonder is the tires are the same size as the ones it came from the factory with and the rear end is the original so what could throw the speedo off that much?

I'm not 100% on this but I believe the speedometer signal is derived from the ABS module on that year. So there is going to be sensors for the front and rear wheels. Is the ABS system working properly?

ABS works like it should. Are there a speed sensors in the trans and the rearend also? I'd like to be able to depend on the speedo but I can rely on my GPS if I have to.

The ABS sensor is in the rear on your model which is also for the speedo. Its standard as they always had at least the RWAL which was the rear wheel ABS and the RWAL and the 4 wheel ABS all us that sensor in the rear for the rear wheel speed and the speedo. It can be programmed for the tire size in the ABS module as we had to do it at the dealer many times. They can check it and make sure it has the right tire size programmed in it first. We could go by tire size or by "revs per mile" if the tire size on the truck is not in the selection on the scanner. Ron