My 1988 k1500 700r4 will not shift out of 1st gear. till about4000 rpm. everything else is ok.

You have a faulty sensor on your transmission must be replaced
with a good technician it should be a quick fix !!

first two things are check fluid,change filter.next is more expense.good luck

your clutch is slipping. it needs replacement.

Sounds like you might have a bit of build up on one of the valve body pistons. I suggest running a can of Trans-X in the transmission for tank full of fuel and then changing the trans filter and fluid. This should clean up everything inside.

Check the adjustment of the transmission T.V. cable. Sometimes they also stick. Try driving it at 30. Shift to Neutral, stab the gas pedal to the floor quickly a couple of times. Then shift back to drive. Then see if it shift normal.

sounds like 3rd gear clutch is out (it might have blew out a lip seal or something)
pull the pan c if there is a lot of black on the bottom of pan (no 3rd = no 4th either) the 3rd gear clutches are engaged along with the 4th gear clutches to make overdrive
if the tcc cable is out of adjustment it mite shift at wot (wide open throttle) max rpm (5000-5500)
if it just 1,2, neutral its probly clutches

The slave cylinder for your clutch has a valve in it, which keeps the clutch from engaging too quickly. The purpose is to keep you from just dumping the clutch and wreaking havoc on your transmission. If you're confident that you don't need that extra safety device in there, you can remove it, just google slave cylinder valve removal. It will give your clutch more of an aftermarket feel.

if it has a kick down cable that could be the problem look for a cable running from throttle to trans that can be adjusted in length

It sounds like your input clutch seals are rolling. Is the check engine light on. It could also be the pressure control solenoid could be causing the problem aswell

You will need a good manual that is focused on the transmission and possibly someone to lean over your shoulder to help you. Clean workspace is imperative. These are rather easy as you are for the most part stacking parts back into the housing when reassembling it. I would say look for a yard trans to put in it meanwhile and then you can learn at your leisure.