it wont be that hard to change springs...
unless you have struts... but dont think any terrano's do on the rear... if the brake pipes are long enough i used to disconect the shock absorber, jack car up until spring falls out, put new one in lower car reconect shock.... BUT i do NOT know if this can be done on the terrano.
failing that.. go to your local hire shop and see if they hire out spring compressors, some do. or buy a good pair of heavy duty ones.(some tools are sold in 3's, these are better as less likely to slip) jack up vehicle, remove wheel, compress the spring with the spring compressing tools, remove spring, decompress spring, compress new spring, fit in place, decompress, check it has seated etc... put wheel back on...
Dont forget, jack car up by body NOT the axle,, you would be supprised how many people jack the axle up and wonder why the spring wont come out... :?
BUT BEWARE, compressed spring can do huge amounts of damage should the compressor tool slip etc, i know of a guy who was killed by a tincorner spring when the compresser tool stripped the threads...
Good luck