They will physically fit, yes, but might not be rated the same as 90 ones.
depends if you want to try to give your 90 a "discovery" feel... might work, might be a bit soft and wallowy, depends on how you load the 90 up, winch,nudgebar etc etc.
For the short time it takes to swap the bits over, i'd give it a go. you might be pleasantly suprised.
disco suspension/ steering fitted my 1987 90 a treat (although i did change the steering box too as the drop link is different, plus mine 90 box was 'freeplay city'..) as for roadholding/ driveability, i think it feels a hell of alot more solid (although i did buy a set of uprated springs all round and replaced any suspect suspension/steering joints) so personaly, i cant fault it another plus point is that you shound gain rear discs off the rear disco axle..
duh.. i thought you meant axles too:doh: so pretty much ignore everything i just said..lol.. although the shocks feel ok (but the disco was fitted with spax or bilstiens or some aftermarket shocks all round anyway..)