Venture 'slipping' into OD

2000 Venture just had a rebuilt transmission about 2 months ago. It runs fine except between say 40-55 it is either like slipping or hesitant it is like a minor whiplash until you get past them speeds. I guess that is how you would explain it. Had Spark plugs and wires maybe 6-7 months ago. It sounds like to me the tranny is haveing a hard time shifting into OD. Any suggestions should I be taking it back to tranny shop or explore other options?

First thing is to check the fluid level.

its full. could the tranny be bad?

Valve body issue....