Motor and transmission mounts

There are a few things that jump out at me..

a. Being a Turbo, you are putting more Torque on the Transmission that was initially designed and it might be wearing out
b. Motor Mount and Transmission Mounts might be worn out or broke.

My Gut is telling me that your Mounts are BAD / BROKE and need to be changed. 3rd has more torque than 4th.

Try this... Find a Friend to do this with you:
Raise your Hood
Put your PARKING BRAKE ON....
Stand on the PASSENGER SIDE of the Van
Have Him/Her Start your Car
Have them put their LEFT FOOT on the BRAKE PEDAL
Have them put their RIGHT FOOT on the GAS PEDAL
SHIFT TO DRIVE and GIVE IT GAS and watch and see if the Motor turns.
SHIFT TO REVERSE and GIVE IT GAS and watch and see if the MOTOR TURN (It will go the other way... TORQUE

If the Motor Raises up more than 1-2 inches, it is a Motor Mount

Remove the wiper arms and the air inlet grille panel. Remove the reinforcement on the drivers side over the motor. The wiper motor and transmission is removed as an assembly and then separated.

Ok what your describing is your Transmission not shifting between gears so low or no transmission oil means a slow car and at risk at damaging it so ckeck the oil level and if you have got full oil it it then you will need to take it to a mechanic.

sounds like a trans mount when they go bad it causes all kinds of shakes , have someone put in gear ,open hood,, watch motor. tell them to hold brake and give it a little gas .Don;t stand in front of car .....see if motor raises up if so replace all mounts motor included. this is a common fix on theses cars.

the over drive which is 4th gear in the transmission has a solenoid to lock in the overdrive. Have that solenoid checked out

to kick down a gear and run smooth is normal, specially for an older car. what you can do is buy a higher octane fuel, as you already did the plugs

run scanner for trouble codes, some auto parts stores will do this for free, change plugs and wires

There is a pressure sol. in the transmission that is bad . This is a common problem that requires removal of the side transmission cover. The transmission does not need to be rebuilt! Talk to a gm dealer. I have sold over a hundred of these pressure sol. and the side cover gaskets to a real gm transmission mechanic that have fixed this problem. GM parts over 20 years.

I would check the transmission and engine mounts but also it may be time for a transmission fluid change. It has a service interval just like oil. 50000 miles

Try running a trany flush machine on it at first and if that doesn't work you may have a bad tork converter clutch.