How high a speed and how was it on a flat suface.If it does it at 45-55 with light throttle it could be tranny shutter.I think it has to do with the torque converter slipping and can cost big money for a rebuild.You can't tell on hills like PaulC said.My 97 7700 had it and i was told by my mechanic to dump it or pay up.I traded at the dealer on a 98 XLT.The 97 was sent to wholesale as I couldn't sell it and screw someone.I was told it will get worse over time.Could also be a COP.Theres a lot of threads on tranny shudder so do a search.