HELP Rear wheels Locked up while driving

[Dogpoor]

Help,
My wife was driving our 2003 H2 with 50K yesterday and manually downshifted the Hummer from 3rd to 2nd while approaching a downgrade. She wasn't driving faster than 20 mph. The rear wheels locked up momentarily and the engine shut down. Sounds like something I have down accidentally shifting from drive to reverse in an old Suburban. I tried to recreate the situation but cannot. Any input would be great. Thanks