You're already set up pretty good for the trip. Being able to air down to about 25 psi (with 4 people) at the trail beginning will make the ride and traction better for you and your passengers. It will be new for them as well and their happiness can make or break your trip. (grin)
Imogene is fun, but it's very rocky and requires picking a good line and going slow. The rocks can be scary looking to first time riders. If your heart is set on Imogene, and you can start at 7am, drive into Ouray, take your time going up Imogene and eat lunch in Telluride. Come back on Imogene and do Yankee Boy, eat dinner in Ouray, or come back over Ophir pass and stop in Silverton headed back. I much prefer Ouray to Silverton and there more stuff and better $ places to eat for your riders.
If your heart is not set on Imogene and since you're coming from the south, take the trail through Silverton to Animas Forks up to Engineer pass to Lake City and then back past American Basin (wild flower heaven) and Cinnamon Pass back to Animas Forks. It's much easier than Imogene but takes all day. It's just as pretty for the passengers but not as technical. This is called the Alpine Loop. Depending on your time there are great side trails coming back to Silverton.
In the Ouray area in summer, you get dirty in the morning and sort of cleaned off by the thunder storms in the afternoon. It's guaranteed. There are lots of people on these trails in summer to help if you get into trouble. Air back up at the end of the trail before hitting 550.
The train ride is an all day thing. You see the valleys. Off road the next day to see the tops.
You have time to order the Charles Wells book. Do yourself a favor and get it.
Read it before you go. You WILL be going back, I promise. 20+ years of consecutive trips taking here.
4x4 and SUV Books : Guide to Colorado Backroads & 4WD Trails (3rd Edition) Cover Price: $34.95
I have all three editions for CO, AZ, and UT. (grin) It will tell you what a driver needs to know.
I could write a book about what to do, but you will leave wishing you had more time to do more trails.
When you get there, keep in mind I did all but two of the trails in my new stock non-lifted 2014 TEP with just BFG's.
http://www.toyota-4runner.org/off-roading/172567-ouray-co-late-june-2014-a.html
No problem. Go slow, look at the scenery, have fun. You'll be fine.
Remember, the rear end on this 5th gen sticks out a bit so just because your rear tires are done, the arse end isn't done yet as it drops down.
PS, all of these trails are on Garmin and your Toyota nav which also uses the same Navtec DB.