01 trooper trans. problems

my trooper suddenly has started slipping when it gets out of 1st. If I engage the winter(3rd gear) button it goes nowhere, just revs. I performed a stall test per the manual:hold down the brake pedal, put it in gear and push the acc. to the floor. In each gear the rpms stalled at 2000 rpm as it should. any suggestions?

How many miles on the unit? Service history? Most common 4L30E problem is a bad transmission range sensor (or mode sensor) TRS/TMS. Do the dash PRND321 lights line up with what you select with the floor shifter? Low fluid level will also cause the issues you describe. Try a search for range sensor here and at the rodeo forum.
G/luck
Joel