With an 87 you are all mechanical if it is stock. Take a close look when you remove the speedo assy from the T-Case and mark it. That plug is eccentric to accomodate other gear sizes and must be put back in properly or you will shread the teeth on the gears. You may have a loose cable into the case or a stripped gear or a bad cable. You are going to have to take something apart to start troubleshooting the problem.
Mark the plug that has the clip or two teeth that hold it in position. It may take a little prying to get it out but the housing and speedo gear will come with it. You can check the speedo head by turning the cable ifyou are lucky you will have enough cable to put in a drill. Have someone watch the speedo in the cab. If all that is good look at the gear is it still got all the teeth with no wear. See how that works the 87 is all mechanical.
89YJ 2.5L TBI, AX-5, NP231 SYE, Ford 8.8, Yukon Grizzly, Tom Woods Shaft, 33" BFG T/As, 4" lift, Warn VR10000s.
AF Jeep Club Tail #184