My 95 rodeo idles ok but when I put it in gear it starts to vibrate. what could this be?

these vehicles are known for ignition coil failure. typical symptoms include: Misfire, power loss, vibrating engine, poor fuel consumption. If the vehicle's engine vibrates, it be misfiring. Is your chk engine light on???

its have a bad idle too? verify sparks plugs, catalytic converter and the flow meter. Have a check engine light on?? Do an ecu scan

Get your trouble codes checked-I had this same symptom once and mine turned out to be a TPS unit.
(Throttle Position Sensor)

Hope this helps.

Replace balancer. You don't notice them until you rev it up.

If your engine is not misfiring but it vibrates too much while idling your motor mounts might be getting worn out. I had a Nissan with bad motor mounts. I was ok in park but when you put it in gear it vibrated a lot. It smoothed right out as soon as you started moving.

This isn't answer but a couple of things to check or do. First has fuel filter been replaced? Start car and put it in gear. HOLD Brake and Give it gas while still holding brake hard!!!! Doess it buck or miss fire? You said it ran bad to highway. Explain Please !!! will answer after you reply.

id look at the gear box first and then the transmission

You actually answered your own question. Your transmission is slipping. The reason that it's vibrating is that it isn't downshifting to 1st gear to start. It sounds like it's either in 2nd or 3rd. The leak may have a lot to with it. I'm afraid you need to find your nearest transmission repair shop ASAP.

I have a 1996 holden rodeo 2.6 petrol, it misses badly on accelleration, I have replaced plugs, leads, distributor cap, and run injector cleaner through... it idles ok,,,,

try checking the fluid level in the gearbox,rodeo dont have a dipstick so u have to undo the plug on the side of the gearbox just like a manual gearbox.if the oil is really low i suggest doing a full service on the gearbox,change the oil and filter is ideal for better results.