87 transmission problems

i have a 87 s15 jimmy 2.8 4x4 had a rebuilt motor put into it at about 150,000 miles now has 201,000 original trans. the problem is that 1st gear slips speedo says its doing 20 it might be doing closer to 5. transmission worked fine on way home stopped for five minutes then it started acting wierd when i drove it home. 1st gear slips, 2nd and 3rd shift fine, and od wont even shift. fluid looked fine didnt smell burnt not sure when it was last serviced had truck for about 10,000 miles. also noticed that around 40-45 mph the rear of the truck would vibrate, someone said it might be my u-joints. ive asked some mechanics and 3 of them said something about the planetary gear or something like being screwed up. transmission will engage but when moving it wont stay engaged it just slips. if i need a tranny i got one i could put in from a 91 s-10 2 wheel drive but i was told i could replace the tailstock and it should work. if anyone can help me with info on this thank you in advance.

I know that although they were both 700R4s there is some difference internally. You will have to take off the rear extension housing, and add your transfer case to it, your right on that. I don't remember if the 87 had the vehicle speed sensor on the transmission, or if it had a speedo cable, that can be interchanged anyways. Either way I don't see any problems with it working.

Check for bad vacuum lines going to the tranny.

This tranny wasn't vaccum operated, it had a TV cable to control line pressure, shift poionts, down shifting.....actully LT brings up a good point, check the TV cable to make sure it hasn't broken off at any point. The little plastic connecter was known to break causing the cable to come off, giving you a shit load of problems.

i doubt its the cable but il check it. one thing i forget to mention was that you can hear whats sounds like a grinding or churning sound from the transmission. what i dont get is that it just happened it didnt happen a little bit at a time it never had a problem i didnt abuse it but it should be replaced soon. thanks for the help