Supposedly the '90 and '91 units are the same, but there can be other differences between CA and Federal models. Get the number from your ECU and compare it to one in a junkyard or other suppliers:

Good Place to Shop, but This One's Overpriced

I bought an '89 unit there for $70 recently. Don't fall for the "core exchange" deal at suppliers. Instead have your old one repaired and keep it for a spare - if you keep your truck, you'll probably need it later. I'm not recommending this company - just passing along something I found on a search:

ECM To-Go

Here's a repair guide if you're handy with electronic skills:

ECU Capacitor Replacement

Here's another supplier:

ECM-ECU

Many folks have the work done by:

Av-Pro

Good luck,
Frank