you can pull the front shaft without losing any oil and put into 4WD with no harm done. just leave the yokes on. tighten the yoke nut on front yoke to remove the play. its a very large nut I forget size maybe 1 1/8"
The real issue here is if you stop moving at all in 4wd. the rear should still turn even if the front quits. (think vacuum actuator) If neither driveshaft is turning then you may have an issue on the mode hub shifter in the transfer case. I agree with randon, try to find a np231 to toss in there they are a dime a dozen, can find in xj and yj, junkyard or craigslist. also make sure your linkage is adjusted correctly so you arent between gears on the TC.