It's an electronic shift is it? Bumping is continuous, or just once?
I suggest it might be the transfer case slipping between engaged and disengaged on the front driveshaft for some reason. Perhaps the traction sensor system? Or something internal to the case. Don't know much about these though
so can't be too specific.
Proud TQE Operator and President of SEFAG - the Society for the Elimination of Ford Automatic Gearboxes
Pacemaker headers + 2.5" exhaust
EF Fairmont interior
Lowered King springs
5 speed manual
Newest mod: EF Ghia wiring loom and dash