1998 Isuzu Amigo shifting and changing from 2WD to 4WD

I have a 2.2L 4WD Amigo, for which the clutch and manual transmission have been acting up a bit. I know the clutch at least needs to be tightened, but there is an intermittent problem with shifting. A few weeks ago, when the engine was warm, it was very difficult to shift - with the gear shift not wanting to go into gear (different gears at different times). The problem seemed to not be so bad when cold, and now I don't feel the difficulty at all. I bought the Amigo used, so I am not sure of tranny service - does the transmission fluid need to be changed out on this manual? Any other suggestions for difficult shifting? Second question - I have been considering a change from 2WD to 4WD, since this model (2.2L) was offered with 4WD. I know that would take some work, but I have found a favorable 4WD rolling chassis from a same-year V6 Amigo (no engine, working tranny and transfer case intact). Would this be a good route? I know that both the 2.2L and the V6 were offered with 4WD, but I am not sure of the exact transfer case compatibility. I would assume this would work. Suggestion? Thanks so much.