88's got a mech speedo.
Start at the transfer case - check where the cable goes in (on the tailshaft housing) - especially if you've pulled it out. Remember that the pinion retainer is "keyed" - so that different speedometer gears can be used. If you pulled it, you may not have put it back in the right place.
You may or may not have the electronic sensor (my 88 doesn't but my 89's do,) and that's one less connection to check.
Worst case, you've had your cable twist apart. You can get a new cable core (the square ferrules are the same on pretty much everything,) and you can pull it out from the top and slip the new one in downwards. Cut the cable to length, and crimp the other ferrule on (you'll usually get a die to do this - it's best to do it in a vice, but you can do it with pliers if you've got strong paws.)
Take the time to go over the whole cable tho - it's just enough of a pain to change the thing - no need to do so if you don't need to.
5-90