My 1994 gmc suburban started leaking transmission fluid on the passengers side now it wont go over 30 mph

I have a 87 chevy suburabn 400 turbo tranny..will it fit into a 94 gmc suburban?

Where is leak at? From the gasket on the tranny pan? If it is it will need to be drained and replaced, might as well replace filter while your at it.
Is it from a main seal? Where engine meets tranny? This may be expensive.
Do not attempt to drive with no tranny fluid, or tranny will burn out very quick.
If it is very small leak, you can use tranny stop leak. If not, you will have to replace gasket or seal.

You need to find the leak (probably in one of the cooler lines or the pan gasket) fix it, and make sure you have the correct fluid level. If you do all that and it still won't go you probably wore out the internal components from driving it with low fluid. Good luck.