Mobo
I would recommend checking the obvious first, the cable connections at the starter and the ground at the engine block which i believe is just forward of the starter. (also even though you tested your battery if you have another one you can borrow and put in there for a day i would recommend it, you would not believe how often that fixes things especially if your battery is more than 3 or 4 years old)
If that doenst help you might consider having the starter gone through. I think the solenoid is pretty much intergrated into the start, so once your that far you might as well have it turned and new brushes and new bushings, ( i suspect your sluggish problems are brushes, that would be my guess off hand if your cables and battery are good) One thing you can do is see if there is a starter rebuilding place around your area, I live near tacoma washington and we have a chain here called "start mart" (i believe that is the name). they have a fixed price of 99 dollars for a lifetime warrenty rebuild of YOUR starter. I suspect for that price they only replace what is absolutly necessary beyond turning and replacing brushes, and cleaning and painting, and the soliniod or bendix if needed, however if it ever acts up again you take it back to them and they fix it back up, I've never had to take mine back its worked fine since.
The other option is just exchange it at a reputable autoparts store but thats probably a little more $$$
You didnt say if your truck is a 4X4, i have a 90 4 runner that is a 4X4 and I could not get the start out through the bottom because of the front differential but if you have a 2 wheel drive you might be able to get it form the bottom you/ll have to get under there and look(also its possible on the 2 wheel drive the starter if facing forward, on the 4X4 auto I have it faces aft)
Anyways the actual changing of the starter is a bit of a pain the first time you do it, I have a 90 automatic V6 4 runner, you'll need a long(2 foot if you can get it) socket extention, and a universal for your socket, i think the bolts are 14mm, anyways there is an access panel in the passenger side wheel well,UN HOOK THE BATTERY FIRST !!!!! remove your forward bash plate from the bottom front of the car, you can access the lower bolt from the front by going up at an angle above the front axel shaft, after you loosen the bolt pull it out from the side through the wheel well access panel, the top bolt you have to get from the bottom back by going up between the transmission and the fram, use the same extention. you might have to unbolt the sway bar at the fram on both sides and carefully let it rest down(dont pull down on it cause you dont want to bend the links attached to the lower swing arms), you might also have to pull the transfer case oil cooler frame attach bolts and let it swing down out of the way, dont let it get damaged either, loosen the upper starter mounting bolt then you can remove it by hand through the wheel well access, the
14mm bolt were the large power cable attaches to the starter is a booger too, but you can loosen it by coming up from the bottom with your socket and univeral, slide under from front and look up and you'll see it, DONT FORGET TO UNHOOK THE BATTERY FRIST, THE CABLE IS HOT!!!! looking in from the wheel well there is a bracket on the tranny holding the throttle cable that comes down from the top of the engine that goes the transmission, you can see the 14 mm bolt holding it, pull it so you can pull the cable a bit out of the way, also there is a bracket that goes along with that lower starter attach bolt that keeps the wiring bundle to the tranny away from the exhuast manifold dont forget to get that back on the bolt when you put things back together, once the starter is unhooked from the main battery cable and unbolted it will fall down a bit and you can tip it up and roll it slightly to get to the soliniod wire, its a plug in connector, press the tab and pull it out, it goes in easy but you need to get to the little tab to undo the connector. believe it or not the starter will come out through that little access cover hole but you have to carefully mess around with rotating while you do it, the first time i did it, it took me several minutes of cussing, the second time(i had to replace the tranny last week so the starter had to be removed again) it came right out, putting it back in is a trick too, cause i couldnt remember how it came out, but if you start it in backwards i think then let it drop down and rotate it up into position i think thats how i did it, it helps if you have an helper who can get under the car with the extention to help get the top bolt started while you hold the starter in place, however if your alone and you can wedge yourself in above the tire to reach you can get both hands through the hole and hold the starter up while you get the bolt started, once the starter is in place its easy to use your extention to tighten up both mounting bolts, you can get under from rear by the tranny and reach way up and get the soliniod wire back on, it just slided into the socket, the main cable can be started on by hand, and a small stubby wrench or have someone hold the cable as not to spin the stud and use your extention from the bottom to tighen the nut. put all the stuff you removed to get to things back on, after a quick test of the starter. the mud flap in the wheel well will need some new buttons to hold it on. also through that access panel just forward fo the starter you'll see the engine block ground bolt and cable.
its not as bad as it sounds, and as i said if you have the 2 wheel drive its probably easier to get to, but set aside the whole morning for it and take your time, your gonna get a bit messy, and if you get it done quickly thats great, but if it takes 2 or 3 hours to remove and reinstall it the frist time dont be fustrated, just keep going and you'll get it, its really jammed in there. good luck john