Gas filled shocks do assist in supporting the weight of the vehicle.
They contain gas under pressure that acts as cavitation damper for the oil within them and also supports the weight of the vehicle.
In the case of a simpler shock with only oil you get no supporting action as you describe.
If you compress a gas filled shock it will return by itself to the fully extended position.
Don't know about the 7XX series but OEM shocks are gas filled at all corners on my 93 945.
Manual load levellers use air and you pump them up to the pressure you want
using the air as a supporting medium.
Automatic levellers use the movement of the shock as a pump to raise the pressure thus increasing the supporting action.

http://www.ohlins.se/pdf/07255-01.pdf
http://www.monroe.com/products/mp_detail.asp?cat=0&detail=Max%20Air

Cheers,
Mikael Larsson