Hmm, haven't heard of this one yet.
Does the vibration feel like it coincides with engine rpm? Perhaps do a brake stall to see if the vibration frequency increases.
How severe is the vibration - 'rattle your teeth', or 'barely noticeable,but certainly there'?
Any fault lights lit? 'Check Engine' lamp, ABS lamp, etc.?
1989 F150 XLT Lariat 4x4 standard-cab, short-bed,5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 210K miles