Both rear turn signals have stopped working, front are fine? Is it the switch?

Turn Signal Lamps
The STOP/TURN fuse in the instrument panel (I/P) fuse block supplies battery positive voltage to the body control module (BCM) for the turn signal lamps. When the turn switch is placed in the left or right position, ground is applied to the BCM on the left or right turn signal switch signal circuit. The BCM then applies voltage to the appropriate front and rear supply voltage circuits.
Grounds for the various lamps are as follows:
• G100 supplies the right front lamps
• G101 supplies the left front lamps
• G400 supplies the right rear lamps
• G401 supplies the left rear lamps

Circuit/System Description
Ground is applied at all times at G201 to the turn signal/multifunction switch. The turn signal lamps may only be activated with the ignition switch in the ON or START position. When the turn signal/multifunction switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the body control module (BCM) through either the right turn or left turn signal switch signal circuit. The BCM then applies a pulsating voltage to the front and rear turn signal lamps through their respective control circuits.
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Did you check the RT STOP TRN fuse 15amp insturment panel fuse box.
Fuse Block - I/P
Left of the I/P behind the side trim panel
If the fuse is good , you could have a wiring problem or the BCM is bad .

Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake is applied, the BCM will then apply battery voltage to the left and right stop/turn signal lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM applies battery voltage through the signal circuit to the transmission control module (TCM) and the engine control module (ECM) indicating that the brakes are applied.
Ground for the stop lamps is supplied as follows:
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps
• G301 provides ground for the CHMSL
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.
Battery voltage from the LT TRN/SIG and RT TRN/SIG fuses located in the rear fuse block is also used by the body control module (BCM) for stop lamp operation.

Turn Signal Lamps
Battery voltage supplied to the body control module (BCM) from the fuses listed below is used for turn signal and hazard lamp operation.
• LT TRN/SIG fuse for the left turn signals
• CTSY/RF TRN fuse for the right front turn signal
• RR TRN/SIG fuse for the right rear turn signal
When the turn signal switch is place in either the LEFT or RIGHT position, ground from G300 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. At the same time the BCM sends a GMLAN serial data message to the instrument panel cluster (IPC) to cycle the appropriate turn signal indicator ON and OFF.
Ground for the turn signal lamps is supplied as follows:
• G104 provides ground for the left and right front turn signal lamps
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps

Do you have a factory scan tool (tech2) ? Your best bet , take it to the dealer .

You would check the socket with a test light or meter to see if you have power and ground. If no power, the turn signal switch would be the next thing to check.

Sounds like you lost the ground to the rear lights.

The turn signal and brake lights are controlled by the turn signal switch.
You did not mention parking lights so I will assume they work.

Could be a faulty turn signal switch or a wiring problem between the dash and the rear of the van.
Do the hazard flashers work on all four corners ?

replace both bulbs,, on the chevy they use the new bulbs with the very thin wire that goes into the socket, they have been having problems with the plastic in the socket getting hot and being partially covered, which makes the connection no good,, try this, with a small, screwdriver or if you have one a small pick tool, gently pry the wire outward from the socket,, Just a very little will do the trick,, then put the bulb in the socket, you should be fine after that ,, if the rear lights are working, then nothing is wrong with the signal switch, since they are on the same circuit. your hazard switch you replaced was probably ok also. if this doesn't work, do the same thing on the bulbs,, to insure a good contact

Replace the turn signal switch. Bad contact inside causing a crossing of the circuits. When the turn signal is applied it should open the brake light circuit allowing for just the signal to work. Doesn't sound like the switch is doing it's job.

YOU HAVE TO REPLACE THE TURN SIGNAL SWITCH.BECAUSE WHEN IT GOES OUT.SOME TIMES YOUR DIMMER SWITCH STOP WORKING.IF YOU GOING TO FIX IT YOUR SELF BESURE TO.DISARM AIR BAGS FIRST.

TURN SIGNALS WILL FEED BACK THROUGH THE TURN SIGNAL SWITCH.THEY ARE INDEPENDENT OUT SIDE ON THE CAR BUT NOT IN THE SIGNAL SWITCH.REPLACE THE SIGNAL SWITCH