You may have rubbed a wire somewhere and made a bad ground. Happened on my 03.

 

That's good to know.

Where was the rubbed wire?

 

I drove the truck the other day and after about 10 miles the transmission started getting really eratic (downshifting when it doesnt need to mainly)

I read in a haynes repair manual that the rear speed sensor is in charge of the speedo and when faulty can cause erratic transmission shifting. I thought I had the problem solved until today. Keep in mind it was throwing codes for the rear and left front sensors.

I just put in a new rear speed sensor and it's still fucking up.

Do I need to get the codes erased?

It recently had a trans swap, do I need to take it to a dealer for some sort of programming?

 

Original post edited for updates.

 

did you find out what was wrong with it my truck is doing the same thing