Hi I am new to this forum but I just got finished fixing my 31' BT Cruiser E450 vibration problem at 45mph. I did some checks directed by my local driveline repair shop.Any one area can cause it. Jack stands under rear axle and ran to 45, nope. Removed axle shafts run to 45,still vibrates, remove rear drive shaft, no luck,center drive shaft, no luck, front shaft remove, no vibration.Sent 3 drivelines to repair shop. Common problem when frame is extened the front shaft is lengthened poorly. Also checked joints and center brgs. all good.