I showed you how to do it.
int DRV2 = map(z, -15, 0, 255, 0); range -15 to 0 => reverse
int DRV1 = map(z, 0, 15, 0, 255); range 0 to 15 => forward
int STRL = map(y, -10, 0, 255, 0); left
int STRR = map(y, 0, 10, 0, 255); right
Yours should be,
servoDrive = analogRead(joyV);
servoVal3 = map(servoDrive, 0, 1023,1000, 1750); // scale it to use it with the servo (result between 0 and 180) FORWARD
servoVal4 = map(servoDrive, 0, 1023, 1250, 1750); // scale it to use it with the servo (result between 70 and 180) REVERSE
// Read the horizontal joystick value (value between 0 and 1023)
servoTurn = analogRead(joyH);
servoVal1 = map(servoTurn, 0, 1023,1000, 1750); // scale it to use it with the servo (result between 0 and 180) LEFT
servoVal2 = map(servoTurn, 0, 1023, 1250, 1750); // scale it to use it with the servo (result between 70 and 180) RIGHT
Again, your map values I dont think are correct. "(result between 70 and 180), (result between 0 and 180)"