Toyota Repair: Shifting problems, vehicle speed sensor, 1991 toyota previa

Question
1991 Toyota Previa 2tz-fe 4cy EFI

Pull the DTC's came up with 21 Main O2s and 42 Vehicle speed sensor. When 1st started runs fine, when fully warmed-up startes to misfire and vibrates in reverse. Any speed above Idle runs perfect no miss however it hesitates and then catches.At the same time the O/D lite comes On,and I have to switch the OD button back on to get rid of the lite. and speedometer don,t work

I know these 2 sensors has to do something with it, just need a jump start where to iniate my diagnosis.

Thanks in advance  

Answer
There is obviously a vehicle speed sensor problem so I suggest trplacing it, you can locate it on the tailshaft housing of the transmission, this should make the speedo work and possibly the O/D light malfunction.
The code 21 has nothing to do with the 42, it's an O2 sensor code that is probably triggered by the engine misfire so the cause of the misfire has to be found.
The misfire could be caused by a lot of things and I can't list them all here, it could be a simple spark plug failure or as serious as a blown headgasket or burned valve, my suggestion would be to have the toyota dealer diagnose this.