The idle speed is incorrect.

Adjust the idle speed. Refer to Rough, Unstable, or Incorrect Idle and Stalling in Engine Controls.

The shift selector linkage is not in adjustment.

Adjust the shift cable. Refer to Shift Cable Adjustment and Shift Cable Adjustment .

The forward clutch has failed and is dragging.

Rebuild the forward clutch. Refer to Allison service manuals.