If it's the shifter shaft seal be sure to lube it up around the inner and outer circumference before installing it. Then use a properly sized deep socket to tap it into place. If the splines on the shaft are cutting the seal, wrap some tape around the shaft to protect the seal from abrasions.
What he ^ said. Having damaged one of these on those splines, I clean the shifter shaft very thoroughly and run my finger down its length looking for gouges or burrs that can cut the seal lips, and run some emery cloth over any I find till the shaft is very clean and very smooth.
Likewise, clean the seal pocket making certain no grit or crud is in there. I use 6 inches of electrical tape, start past the splines and wrap the shaft and then the splines til it overlaps the end, then I lightly lubricate the tape, shaft and the seal. Carefully work the seal over the tape, slide it down the shaft, and do your best to get it started squarely so the leading edge doesn't get caught or folded back as you insert it. Start inserting with your thumbs if you can, be certain it's started in there and square to the pocket, then drive it.
A few years back I raided the cut-offs box at my local Ace, and took 2-foot sections of several sizes of PVC pipe, and I use those (cut to the ideal length) with a mallet as seal drivers.
