tranny Slip

I just got done putting my rebuilt 4.0 back in and after a few minor setbacks its running great. I bought this with the broken piston skirt and only drove it a few miles. with that said now when i back out of my driveway and put it in drive it hesitates for a second before it goes into gear. once its moving its all good so i don't really know if it was a problem before. I checked the fluid just now and it is a bit low but wouldn't think enough to make it slip.any ideas?

Thanks Haney