im looking to start collecting parts for my 4wd swap on my 98 xj. how can i know if a trans is good before putting it into the jeep? or is there no way? im a broke kid working minimum wage and dont have the money for new parts so i was going to use car-parts.com. just dont wanna get screwed

No way to know how good a part is unless you test it. To swap over to 4wd you would be looking at trans, t-case, front axle assembly, both drive shafts, shift linkage for t-case. You might be better off finding a 4wd Cherokee and sell the 2wd one. By the time you round up all of the parts and pieces and the time to do the work, you could be into it by 1-2 thousand. Your best bet could be to find a rotted out 98-99 Xj that has good running gear at least you could test it out before you buy. and that you could find for a few hundred.

trans could run you 150-500
t-case 50-150
drive shafts 20-50 junk yard
linkage if not on t-case 20-30 junk yard
front axle assembly. 3.55 gears 200-300
fluids, possible small parts, tools... 100+

I'm sure I am missing some things there are plenty of people that have done this swap.
Hope this helps

I would second what dan's saying. I recently bought my 95 XJ with a solid tranny, engine and pretty decent body for $800. I had to put a muffler, wheel hub, sway bar link and some body parts and had the rear brakes/e-brake completely replaced, so I now have a working 4wd, inspection passing XJ for roughly $1600 (not including taxes/fees/general raping by state of MA).

Unless you directly test the transmission (driving or using extremely expensive professional tools rarely found outside a dealer/tranny shop), OR buy a new/newly rebuilt one, it may or may not function and you're stuck with the word of a seller. A junkyard/used parts retailer may give you a guarantee, but typically that's a "if it doesn't work we'll take it as a return or if it fails within 30 days".. which means you could spend many hours installing and removing if it's bad.

Buy a donor vehicle. If I have to do it again, thats the route I will go. I am in my swap at least 3k but the majority of my parts were new. Lot cheaper and easier just to find a running driving rusted out bucket for under $1000 and call it a day. Just make sure its the same year as yours.