05 Express shudders
 

My 05 Express 3500 has developed a shudder when it goes over humps and dips in roads. Around 30-50 mph and a couple of times it shudders pretty hard. The van has Stabilitrac but I am not sure if this is how that works or if this is a malfunction of Stabilitrac. On smooth roads it rides smooth and on rough roads it doesn;t shudder all the time, just here and there.
The dealership checked all of the linkages, wheels bearings, etc and found nothing wrong. This van does have 17" aftermarket wheels that are 8" wide versus the 16" by 6.5" stock rims. The dealer said they think it could be the wheels but I am hesitant to this theory since the van rides smooth on good roads, highway, etc.
Returning to the dealer today. Meanwhile, suggestions are welcome.

Thanks, Greg