BAD vibration at 15mph, deflated rear air bags. Help!

[transit360]

Greetings all. I first want to apologize as my bus is not a
Wanderlodge, but I am a Bluebird enthusiast and this is the closest
group I can find!
My '98 Bluebird Q-bus diesel pusher has a terrible vibration
coming from what FEELS like the rear. It started after I hit a speed
bump unexpectedly way too fast. At least, I think that's when it
started.
Description: No vibration until about 10mph, frequency of vibe
increasing until the worst at 15mph, then gradually gets better. I
haven't taken it over 35mph yet...limped it home. There if I put the
tranny in neutral while driving or if I rev the engine, so it's
definately in the drivetrain. The bus DID NOT have this problem
earlier today while driving, even on the interstate.
What it feels like: Like you going over a wood bridge with
planks perpindicular to your travel. High speed vibration...so I don't
assume it's tire related. Kinda feels like my Jeep when the front
driveshaft was out of balance at 60mph, but on the bus it occurs at
low speeds.
What I've checked: Of course I checked the driveshaft first. All
seemed fine. Compared it to my other good bus and was the same. THE
ONLY THING I noticed different were that my rear air bags were
deflated entirely, even though my air gauge was fine and all air-
related accessories were fine. Could this cause my vibration?
I started the bus up and aired it up all the way and re-checked
bags. The same...deflated. NO audible leaks, either. I don't know if
the air bags were deflated before I hit the bump. It's possible they
could've been like this awhile.
As a side note, my air-moisture ejector constantly vibrates
under your feet while standing above it inside the bus. It constantly
build pressure until it pops off. The air ejector stops vibrating
while the air is actually ejecting, then starts right back up again.
I'm wondering if my air bags have been like this for awhile and this
is why the ejector valve has been doing this over the last few months.
So that's it? Sorry for the long post. Any suggestions?

Steve
Two '98 Bluebird Q-bus Partybuses