1988 bronco automatic transmission

I would service the transmission do a filter change and flush on if

standard or automatic transmission. and does it do it when driving, or is the shifter bad at a stop.

There is a cable that goes from the transmission to the Throttle.  If this cable is broken you will not shift out of first.
This is for an automatic transmission.

There are shift solenoids in the transmission and apparently there's a problem them. I would get it to a trany shop and have it checked right away. If you continue to drive it you will ruin the transmission. Are sure you put the right computer in the vehicle? If you put a computer from a Bronco II with a manual trans in a vehicle with an automatic trans, the trans won't work right.

your output seal on the tranny or input seal on your transfer case is bad. you have to pull your transfer case to tell. this is a pretty straight forward job. block wheels. put transfer case in neutral. drop front and rear drive lines.diconnect transfer case shift linkage. there is a short commponent connecting the trans to the transfer case. the bolt on either end face each other. remove the bolts to the transmission. the transfer case is heavier than it looks, i suggest getting help. pull back on the transfer case until it will drop down. it will pull out a bit because of the trans output shaft. replace what will probibly turn out to be the tranny output shaft seal and reassemble in reverse order.

is the idle speed correct? do you shift into second before trying to engage reverse?, is the clutch ok, this is most likely not the trans at all but one of the above issues, the clutch is my guess.

get the transmission solenoid checked out

try shifting it manually from low,let rpms rise,then 2nd,let rpms rise,then to drive and see what happens.It could be bad drive clutches in the transmission but could be a sticky valve body in the transmission,if it does work by shifting manually add 1\2 pint of dot#3 brake fluid to the transmission fluid to assist in cleaning the pistons in the valve body and seals and to help lubricate parts and seals

When was last fluid & filter change?