700R questions

[Chrispbrown36]

First the stats...1987 Chevy V10 with a TBI 350, 208 transfer and 700R4 (this is what is listed on the invoice but I have not yet confirmed this by checking it out, but will today) I am wondering if something needs to be tightened up on my transmission. When shifting from park to overdrive the indicator sort of slips easily between overdrive and drive. If I shift straight to the overdrive position the truck remains in neutral, but if I shift over to drive it will kick into some sort of gear and then I can slide it back to the point where the indicator is pointing at overdrive. I honestly can't tell if I am in overdrive or just drive. 1st and second engage like normal, but there just seems to be some slop between overdrive and drive. I am really wondering if maybe someone hasn't put a non-overdrive tranny in it at some point and that is why there doesn't seem to be a difference between the two. Is there something I should look at tightening up? Linkage or otherwise? or am I possibly looking at a failing tranny? There is no slipage of any kind while drive.
Thanks in advance for any info.