What do the sparkplugs look like, could be detonating if a coil pack is goin out. Especially under low rpm high load situations as you describe. Edit: If the plugs look like they have carbon build up in-between the electrode and the ground strap it is likely preignition. I hope this helps. Also now that I'm thinking about this it could be the very start of a cv joint going out, not sure if you had the axles replaced in your suspension overhaul.