A4ld no overdrive problems

I have a 87 Ford Bronco II and replaced the trans with another 87 A4LD and it shifts through all gears just fine but when I shift it into overdrive it doesnt want to shift into overdrive.Was just wondering if anybody might be able to give me any tips as to what might be causing this thanks.

Also when im driving down the road and let off the gas the rpm's drop off to 1000 like it's in nuetral then when I give it gas it feels like it kicks back into gear.Any help on either of the issues would be great.

sorry man I don't have any A4LD experience, check in the transmissions, transfer cases & differentials section and see if you can find something in there.