My 2538 wanted to do that too, especially going downhill. It's possible your range gear shift is out of adjustment. There should be a bolt holding the lever to the linkage under the seat deck that you can access, loosen and re-position.
One of my problems was that the slot in the seat deck wasn't long enough to let me move the range lever far enough to totally engage the range gear. Took my grinder and made the slot longer, and solved my issue. Not sure your's is the same, but something to check out.