I just purchased a 99 trooper and the "check" light on the TOD is flashing. The floor shift 4H works fine, four wheel drive engages. However when using the auto 4 wheel drive button when the rear wheels slip the front diff never engages. After researching I have found it may be either the vacuum selenoid on the front diff or possibly a speed sensor on the transfer case. Because the 4x4 engages in 4H does this eliminate either situation or is the enagaging of the auto 4x4 completly unrelated to the floor shift?