Locjsmith and others who read this response. You may not want to hear what I'm going to say but it needs to be said. Please do yourself a favor and don't throw $$$$ at another persons suggestion because he did and it fixed his problem. There are a log of guys on these and other forums that do know what's going on and can diagnose issues even when we can't actually experience your symptoms. It's OK to get ideas but get more than one idea. Most people can direct you down a path based on your symptoms but every coach is different and what fixed one may not fix another. It bothers me when people spend $$$ and it does not fix the problem.
So what can you do??? Ask the questions on these forums and get suggested solutions. Contact your repair place and explain. Listen to their solution. If it sound like they are throwing parts at it then they are not the ones you need to use. Ask them if they can test the part?? Show you why they think it's a bad part. If they can't then leave. EVERY PART, SYSTEM, ETC ON A VEHICLE CAN BE DIAGNOSED. Evey part has a: name, location, function, symptoms when they fail, and ways of testing the part or system.
Replacing a suspected bad part with a known good part is what I do generally as a last resort. Electrical parts sometimes can't be tested without some expensive test equipment. If a tech does not know what he's doing that is sometimes the first step. It's no skin of his wallet but yours.
Now your vibration. Did somebody ride in the coach and pinpoint the location?? Did they lift the coach's rear wheels, put it in gear and accelerate to try and locate the vibration. I've done that before and it works. Your center support bearing if bad enough could have been visually moved and you would see the slop in the bearing. Also, just because a shop said they balanced the drive-line does not mean that it was done correctly. I'm not saying it was but anything is possible and since everybody thinks it's balanced they just skip past that as a possible reason. You can take a hose clamp and add a wheel weight (2-oz) to the drive shaft and see if it changes the vibration. Change the location of the weight and move it from shaft to shaft. I realize that you are adding weight and that just might cause a vibration. On the other hand you might move it into a place that reduces the vibration then you know that there is still an imbalance somewhere.
Brakes won't cause an imbalance unless you lose a weight from a drum. Disc rotors are drilled to balance them most of the time. Sometimes they will add a weight into the vented part of the rotor. If your vibration came all of a sudden you might have lost a weight but the spot it fell off of would be visible. If you have rear drums and did lose a weight a shop is not going to look for a weight that fell off. So when they did the brake work they may have just missed it or didn't look for it.
I think you said that the tires were balanced. If not then you could have thrown a weight. That's also easy to spot because it leaves marks where it was. Maybe a piece of the tire came off causing an imbalance.
If it's related to road speed it has to be something that changes in RPM as you increase and decrease road speed. Those items are: tires, rims, rotors, drums, and drive shaft.
If while it is vibrating you put the vehicle in neutral and allow the engine to return to idle and the vibration goes away then it is related to engine speed.
TeJay (Tim) Auto Instructor 35 yrs (4-yrs USAF) Liz: RN/ WBGO 2014 Vista 30T/ F-53/ CHF/5-Star/Koni/Centramatics * Bella- Golden/Cocker mix & Louie-The cat / All Retired