Lots of things to consider... Unless the truck is really old, it should have a connection for a diagnostic code sensor. The van will definitely have this... a plug under the dash, where a code reader can be attched. Code readers are available for about $60... From the code, best to go to an online forum for your vehicle. A little searching should bring up a good forum with mechanics that can help, usually with good instructions.
One small point before trying to remove the sensor yourself... (wherever it is located). The temperature should be at least $180 degrees. Sensors are installed with a special "glue" that needs to be heated, to release the threads.
Sometimes, resetting the light will solve the problem, especially if it initially came on after long terms of idling. There are or can be other reasons for the light, sometimes caused by faulty or clogged injectors... and even from the sensor itself... which sometimes can be cleaned. In no case use regular cleaner such as a brake cleaner, but an electrical cleaner as available at Radio Shack, or a good automotive store. Cleaning can be a very delicate process.
The location of the rear sensor may be a problem... in some cases requiring removal of the heat shield, and undoing the rear engine mounts to gain access. Every car is different.
Re the vacuum leak... spray brake cleaner or soap on the vacuum connections help in diagnosing this.
My own car has an onboard diagnostic center which allows for pulling codes. I have a recurring O2 sensor problem, which enriches the fuel mixture and reduces gas mileage... (the fuel sensor works with the on board computer to change richness.) if I reset the sensor, the mileage improves, until the next time... sometimes 1000 miles later. The shop "fix" can be from $500 to $1500 and more, and according to the forums, may not fix it, even then. (A known problem on the 1996 SLS).
I'm not a mechanic, so this is just shade tree stuff, but when it comes to the big bucks to fix something that is hard to understand, I feel your pain.
good luck...