Transmission question on 1993 Nissan Pickup (D21)

I just bought a 1993 Nissan D21 Hardbody the person that I bought it from told me that the transmission was rebuilt but at times feels like it slips out of gear. it is a automatic transmission but when I drove it home it didn't seem to slip that much to me. I just want to know what I can do to fix the problem and if I have to shell out a lot of money to do so. any help would be appricated.

Asked by for the 1993 Nissan Pickup (D21)
ck your fluid level and condition 1st thing.
could it be the transmission filter needing to be cleaned or changed? would that cause it to seem like it slips out of gear at times?
maybe but unlikely
maybe but unlikely