First thing I'd do is test the engine braking from about 25 mph. There should be MORE engine braking in manual low gear than in manual second, and essentially none in "Drive".
Try this first, either way, your looking at having yours rebuilt or getting a known good trans installed
98 Regal GS - 279k, on the race to 300k
99 Silverado Z71 - 206k, BGA express
03 Grand Prix GT - 87k, Driving straight down the road again
02 Malibu - 166k, beater with a heater
03 Grand Prix - 224k, parts car