I've got a 2001 2500 4x4 with 262,000 miles. Stock except for 4" monster exhaust, K&N filter & tube, Banks intake. When i take off from a dead stop the 1-2 shift feels like one big neutral. I can take off hard, then let off the gas, let 'er fall into 2nd, gas it again, she goes fine. If I take off slow or the tranz downshifts at low speed- big slip.When it starts slippin' I can floor it, pin the tach, no grip. As soon as ya let off, it goes into 2nd & pulls like a mule. Any advice would sure help out.