My 1989 Plymouth voyager 2.5 4 cly Turbo... is stalling after a short drive (wont accelerate without stall) and dies when idleing , Check engine code said it was my speed sensor... would this be the problem?? And where would it be at?

It could be. They are normally located on top of the transmission, close to the firewall. They are usually attached with two bolts.

How could I tell if it needs to be replaced..?

VSS code being thrown is likely enough evidence to replace it. You also want to go over the ignition system--coil, plugs, and Ignition Control Module.