magoz on "Define the category used in get_next_post"

Hi,
I have posts that have different categories. Some of them share same categories in addition of having different ones.
Example:
Post 1 has Categoy 1 and Category 2
Post 2 has Category 2 and Category 3
Post 3 has Category 1 and Category 3

I need to be able to choose which category is going to be used in next /previous posts.
Example:
For Post 1 use Category 1 only. (which will reach post 3 as next post, not Post 2).

I checked the function reference in WordPress codex and it can't be done with the parameters of /get next post. You can only exclude certain categories, use the current category or specify a taxonomy.
I need to be able to specify the category of the next / previous post by myself.

My idea is to get the category current printed in the url.
I'm using the following code, where:
$categoryslug is the category slug printed in the url that I want to use to tell WordPress which category will be used in next / previous posts.
Code:

<?php $url = 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['REQUEST_URI']; ?>
	<?php if (false !== strpos($url,'?c')) : ?>
		<div id="nav-portfolio">
			<?php
			$categoryurl = explode('/', $url);
			$categoryslug = str_replace("?c=","",$curl);
			?>
			<div class="prev">
				<?php
				$prev_post = get_previous_post();
				if (!empty( $prev_post )): ?>
				<a href="<?php echo get_permalink( $prev_post->ID ); ?><?php echo $categoryurl[sizeof($categoryurl)-2];?>/"><?php echo $prev_post->post_title; ?></a>
			<?php endif; ?>
		</div>
		<div class="next">
			<?php
			$next_post = get_next_post();
			if (!empty( $next_post )): ?>
			<a href="<?php echo get_permalink( $next_post->ID ); ?>
				<?php echo $categoryurl[sizeof($categoryurl)-2];?>/"><?php echo $next_post->post_title; ?></a>
			<?php endif; ?>
		</div>
	</div>
	<?php endif; ?>

Getting the category slug from the URL works well, but I need a way to tell WordPress to use only this category. Maybe working over this part?
$next_post = get_next_post();

Any idea of how to call only a specific category in nex / prev post function?

You can check the website I'm working on here:
http://magoz.is/_beta/follow-your-dreams/?c=personal/

Thank you very much in advance folks!