TENS units can be stronger than you can stand, but the current stays pretty close to the skin. EMS gets deeper, but is better for muscles than nerves.

Interferential units are strong enough and the pulse frequency and wave-form is adjustable for both muscles and nerves. They get much deeper than TENS, EMS, or the ReBuilder. They're not expensive, but the supplies can add up. I got my interferential unit at>>>>> oops, I must have deleted all the links and email to it. This is the unit MrsD recommended, and several links to companies that sell it. My physical therapist uses the IF 4000 when his big unit is in use. It works great:

http://www.medicalproductsonline.org...ential-if.html
(high price - pay by Paypal*)
http://www.med1online.com/pc-12508-2...nit-77719.aspx
(low price...but no instruction manual, and a different brand name. It looks the same as the IF 4000)
http://www.tartangroup.com/store/ite...1&ITEM_ID=1779
http://fitnessproductsplus.com/ecom....25&gotopgnum=1
(good price for IF-4000, with manual and case - pay by Paypal*)
http://www.amazon.com/Channel-Interf...3064954&sr=8-2
http://www.egeneralmedical.com/dual-...tens-unit.html (web site of the company that supplies Amazon...selling the same unit for $30 less than Amazon - pay by Paypal*).

* Paying by Paypal offers very cheap return insurance if you feel more comfortable with that, and Paypal gives you an extra layer of security since you don't have to type in your credit card number again after you're registered with them. That being said, I've had nothing but good experiences with online medical supply vendors.