I have a 1977 CruiseAir Motorhome with stock Chevy 454. Replaced the tranny 2 years ago with junkyard. Has run fine until last week when developed strange vibration that seems to be speed related (60 mph & up). Power doesn't seem to be affected, and isn't hard to handle. Just a "pulsing" vibration almost like crossing bridge grates (comes and goes with a frequency that increases with speed). Any ideas?

Possibly Ujoint going bad or driveline may need to be balanced-could also be a tire out of balance-another idea could be loose flex plate on engine or torque convetor bolts loose!!! :)

Tank an oil sample from trany see if there is any signs of metal cuttings or the texture of sediment looks like metalic paint, thi may be a sign of a failed bushing or bearingand components may be out of alinement and touching housing giving you tha pulsing unit may need overhauling or replacing a junked in good condition whould but cant garenty service life as with a rebuilt or exchanged unit