I have 2001 jeep cheerokee with 4wd and 114000 miles. Yesterday morning drove in heavy rain to work. After 8 hours of sitting in lot went to back up and heard a thunk noise. Thought I hit something pulled up and stopped to look nothing . Backed up again slower wheels seemed to stop. Pulled forward no problem after driving home backed up again no noise. Could it be brake problem. Thanks

I think it was just your drum brakes that got wet and after sitting they stuck while drying off.
Sounds like a no big deal. Brake surfaces will form rust in no time after rain.

Thanks> Do you think it would be wise to have the brakes checked?

Always a good idea to have brakes 'checked'. They're a safety item aren't they? If it's been a while, then most definitely!
You don't need to have the work done then and there, unless of course, they need immediate attn.

Well it was brakes ! Had front and back done . Now no noise and no sticking.

Excellent!