I want to convert my standard diff to posi Can this be done? If so What do i need

It sure can. Just make sure you get the correct posi. Since you said you have a 3.73 open rear, I'm assuming it's a 10-bolt rear and that those are 3-series gears. A 3-series posi should go right in if it's the correct one. Also, make sure you know what axles you have, 26 or 28 spline. SLP has a take-off zexel-torsen (3-series) posi for $99 that will fit 10-bolt rears with 28-spline axles. I bought one for my '92 RS (2.73 open rear) and plan to swap to 3.42's sometime soon.

If you're happy with your gears, an even cheaper option is to get a PowerTrax coversion unit. You can install it yourself and avoid the cost of getting the gears set up, which you have to do with any other change that involves removing the carrier.