Is there possibly a high idle switch installed? On the '96 and newer models, if the high idle switch is engaged, the engine will revert to normal speeds once the brake pedal is pressed and/or the gear selector is moved out of park.
Here is info on the high idle modifications:
http://www.kennedydiesel.com/FastIdleSetup.cfm
Otherwise, I would suspect a failing PMD/FSD. I have personally seen them fail where there is idling/RPM issues, and have heard of them failing and going to wide open throttle, which can be scary.
Casey
1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter