Speedometer goes to 0 and trans downshifts

you did not ask a question, why is that? just a code.
which trans, regular or CVT? there are 3 transmissions. and m/t
the Trans chapter shows 7 trans. and 4wd options.???
VIN code A,C or G? (tranny code is in VIN)????
p2 is a factory only code.
Downshift Switch Circuit Low are asking how to do all the FSM steps. to diagnose this.?
you reset it and it came back
are there driving symptoms??????????

the set is.
P2779 Downshift Switch Circuit Range/Performance
P2780 Downshift Switch Circuit Low
P2781 Downshift Switch Circuit High
P2782 Downshift Switch Circuit Intermittent/Erratic

the key to understanding the cause is to understand the test.
how is the test done, (monitors) and what does it actually measure.
a real fsm will tell you if the LOW means, voltage at solenoid is low.
or RPM was too low.
see?
and is key to the cause.
i dont have the real FSM, and online shows nothing.

There's no such thing as a downshift relay.Trans downshift is cable actuated.Go to the end of cable at the carb/throttle body,open the throttle fully by hand and depress the black button in the cable connection block,the cable housing will pop and when your driving will automatically reset to where it needs to be.Your done.

run the fault codes and check for TCM problems as it controls the gear shifting, speedo and rpm's gauge.

You are experiencing first stages of trans failure, and the 4l60 has a few

there is no vacuum line for reverse. the transmission uses fluid and silanoids inside for reverse. if reverse isn't working the low/reverse band is blown. you can test it by driving and manually downshifting into 1 and if it downshifts through all gears but wont downshift into 1st (feel the truck slowing down from engine braking) then your band is broken. time for a trans rebuld

Best Manuals I honestly suggest you to not waste money on repairs. They're never satisfying and mechanics don't keep you in the loop very much. Try manuals and doing it yourself, you'll know better, and can also judge better.

You dont want to start buying 250 dollar parts , that you dont know you need.You need to hook a scan tool to this thing and see if the downshift is even being comanded. When you come to a stop the scan should have 0 output rpms and should say the trans is in low gear.If you have say 14 rpms showing on the output speed sensor at a stop the the computer will think the vehicle is still moving and not allow for a downshift.This trans does not have govenor solenoid and sensor like the earlier jeeps did.Its electronic controls are more like the mini vans with the 604 trans. I have seen mini vans have a downshift bump and / or no downshift because the plugs wires were too close to the wiring harness and creating a falseoutput speed signal. It will pay to have this Transmission diagnosed by someone with knowledge and the tools.

most cars including yours, you have a speed snesor on the gearbox, which tells the computer (ecu) and transmission how fast you are going....the fact that you have included that the speedo is erratic, is telling me that the sensor needs to be replaced, as it is telling the ecu (which tells the speedo) the incorrect signal as well as the transmission......hopefully, easy fix

this reminds me that cars are like humans IN A WAY, the same symptoms may lead to a cause, the car not downshifting and the erratic speedo are the symtoms, just like a runny nose and cough are symptoms, and the sensor is the cause of the faults, where the cold is the cause of the faults in us....how cool

speed sensor needs replaced

Educated guess: Sounds like a shift solenoid or a speed sensor gone bad. Search a Ram truck owner forum (start witha  Google search) to see if it is a common problem.