Type of transmission

Check the fluid level. You may want to consider having the transmission flushed. (I recommend BG Products flush system.) Properly flushing the trans removes any varnish the may have built up causing erratic behavior.

what transmission is in the truck ! one is due to an electronic selonoid the other is due to transmission pressure in in overdrive -- also is it over drive that does not make it, or the lock up convertor ?? I reccomend you take the truck to the best transmission shop in town or a GM dealer for an accurate diagnosis of the transmission malfunction(s).Not a time to make an uninformed or an expensive mistake !! have a good day !!

Have the tranny fluid changed. may want to change the filter at the same time.

no. in the late 80's chevy switched to a electronic automatic tranny called the 4L60E, that is the tranny you should be looking for. the only bad part, its more expensive.

It will work but you will have to modify the crossmember and driveshaft and the computer. The 4l60e is an easy trans to recondition

sounds like your forward clutches have burnt up. have transmission rebuilt or replaced.

be 1988 and Im sure it has high miles, sound like worn transmission, transmission flush and replace it with new fluid might help some. get use tranny from a junk yard better and cost lot less than rebuilt. being GM mechanic for many years i see transmission on a truck with 100k tend to have leaks or need rebuilding. again i see some have better luck but rear.

is check engine light on? check the tps.

IHave the pan dropped and the filter check/replaced and the sediment evaluated.

It will need replaced or rebuilt, it has a bad 3/4 clutch