955 st. can i stiffen the rear shock to give greater resistance??

no the bike is not fitted with a ride height adjuster you can only stiffen or soften the rear shock absorber preload on the spring or drop the fork through the yokes.you can on the other hand buy either shorter or longer dog bone links for the rear suspension linkage to lower or lift the rear suspension

Replace the spring with a heavier duty should do the job.
Have you tried adjustments in the spring load to stiffen it up.
The rear shock on your Yamaha V-Star 1100 motorcycle assures that the rear wheel is always in contact with the ground and makes your ride much more comfortable by absorbing road vibration and shock while you ride.

But your shock needs to be adjusted for different riding styles, so depending on whether you ride fast or cruise slowly and how quickly you take turns, you may want to adjust the shock to get the most out of your ride.

1

Remove the passenger seat by loosening and removing the rear passenger seat mounting bolt with a socket wrench.
With the passenger seat removed, you should see two larger rider's seat mounting bolts.
Remove these bolts and pull the rider's seat off of the bike to expose the internals of the motorcycle. 2

Detach the quick-release clips of the ignitor unit found just beneath the seat using a screwdriver. Pull the ignitor unit up and to the right of the bike to move it out of your way.

Remove the mudguard quick-release clips in the same fashion, then pull the mudguard out of the bike to expose the rear shock adjuster. 3

Look for the small numbers on the rear shock adjuster.
On most 2002 V-Stars you should see the numbers 1 to 9 printed on the shock adjuster.

These numbers give you an idea of the current setting of the shock.
The higher the number, the stiffer the adjustment. 4

Loosen the shock by turning the shock adjuster counterclockwise to a smaller number using a spanner wrench.

This makes the shock softer and more forgiving when riding over bumps and uneven pavement, but will feel slower in turns and accelerations.

Tighten the shock by turning it clockwise to a higher number.
This makes the shock stiffer, increasing road response in turns and giving you a better feel of road conditions at the expense of comfort. 5

Replace the mudguard, ignitor unit, rider's seat and mounting bolts, and the passenger seat and mounting bolts.

Take your bike for a ride to feel the difference a turn of the shock adjuster makes.

You can install helper springs in the rear to help stiffen up the ride a bit. Also look at the tie rod links and make sure they are not dry rotted.
Worst case you can add an aftermarket air ride system to help with the extra weight.

Around 20psi with shock damping on position three gave a good ride in conjunction with 8psi in the linked front forks; for serious street racery another 10 psi in the shock and max damping helped stiffen things up.

They have built in steps to tighten the spring load. A special tool helps, but you can with enough loud words, kicking and biting, get it to move. If you can get the bike up with the shock extended or the pressure off the spring it will be easier. Hope this helps.

TAKE YOUR TOOL AND TURN IT CLOCK WISE THAT WILL STIFFEN YOUR BIKE.

yes as far as i know they are all interchangable as long as they are the same year

Its likely that you got so used to the soft ride you had with old worn shocks that the new ones feel strange. You could buy a more inexpensive shock to restore that "old" feeling, but I don't think you really want that. One thing you should do though is to make sure by having a couple of people bounce the rear, that the shocks are not binding or are too long and bottoming out part way through the suspension travel. There is a part # stamped into the bottom of the shock just above the weld ...double check that part number to make sure you have the correct ones. Also check the standing height of the back end (compare the height of yours with others you find in parking lots...I have a 93, which measures 171/2" from center of rear wheel hub to edge of wheel flair.)
If standing height is too low, shocks are not working at the center of travel but at the lower part. (likely you would have noticed this more with the old shocks though). Additionally, there are adjustable shocks available that you can "tune" to your comfort level...they usually have three settings ...you choose the one that suits you best.
Hope something I said will give you a cure or an understandable reason for what you are feeling.
Don't forget though that a Jeep is essentially a small truck, as such, it's going to ride like one!