you should have a 6.7" diff i assume, unless you have a very late update diff ('87) with the update axles - in which case you still have a 6.7" diff, but the spider gears will be a bit different.
you have several options - buy a 4.1 out of a t18/ta22 etc. buy my 6.7" 4.3 buy a new one from toyota
you will still have to do some retrofitting to get it going if you do have an update diff though