Yeah, that definitely is not good. Sounds like something is very off balance, but because it doesn't happen in idle it makes me thing it has something to do with the clutch, possibly a bad pressure plate? Call the shop, complaint, and have them pay for towing back to fix it for free. Worst case you'll break the crankshaft bearings and the transmission, depends on how bad it is.
2003 Mustang GT
Intake & Exhaust: K&N FIPK CAI, Mac O/R H-Pipe, Custom Catback w/ Magnaflow Magnapack Mufflers.
Suspension: H&R Super Sport Springs, KONI STR.T Shocks & Struts, J&M Caster Camber Plates, Steeda Bumpsteer Kit, UPR Elite LCA's & UCA's.
Performance: BAMA 93R Custom Tune, FRPP 4.10 Gears w/ Terminator spec T-Lok.
Planned Mods: FLSFC, Eibach Sway Bars, Throttle Body, Plenum, Rims.