A quick and easy way to make your rear tire work like new is to cut the knobs square again. If you have a knobby knife, you can use that and most say they have good results using it. If not, you can purchase an inexpensive decal remover from harbor freight. It looks exactly like the knobby knife, but is cheaper.
Last option I know of is what I did 2 days ago to my rear tire before a local harescramble. I used a 4-1/2" angle grinder to re-cut the leading edges of the knobs in the drive direction. I used a 1/8" abrasive cut off wheel on the grinder. Wear safety glasses and do it in a well ventilated area. There will be smoke and hot pieces of rubber coming off the wheel. Last, remember to take it a little at a time. Don't go too deep or you will ruin the tire. It took me around 30mins to do the rear tire, I was in no rush.