I noticed some quite play in the rear U-joint, so I took the drive shaft to a shop to replace the U-joints and get the shaft balanced. There was no vibration before but I thought I should get it fixed before it fell off. But after I mounted the shaft I noticed a vibration that starts at 45-55 mph. The vibration goes in and out once a second or so. Is it important to mount the shaft in the exact same possition, both at the transmission end and rear end? The shaft is a two part with rubber bushing towards the rear end. The shop says that I'll have to get it rebuilt, but I don't understand that since it realy worked ok before. I read that the rear transmission mount could cause vibration?